I assume the "driveshaft tunnel" is there because it's a shared platform and that was easiest...but yeah, as long as it's there, they put batteries inside as tm1v2 notes.
I have been driving a Model 3 Performance for 3 years, and I just drove a Polestar 2 a couple of hours ago. I largely agree with
tm1v2's detailed assessment. The biggest difference, for me, is that the steering felt kind of numb as ForDesign said. But then, tm1v2 drove a Polestar with the Performance Pack; while ForDesign, like me, is in the Seattle area where they currently only have a car without the Performance pack for test drives. So maybe that Performance Pack does make a very noticeable difference.
Here's my take on the cars, if anybody wants more input on comparing the 3 to the Polestar 2:
Exterior: I think the Polestar 2 looks better. But not a lot better, and this is obviously subjective. It's about 2 inches taller and 3 inches shorter; from photos I thought it was taller than that and ingress/egress might be easier, but I didn't really notice a difference. At least, not in the front - the shorter rear doors and lesser rear legroom in the Polestar made it harder to get in to the rear seat.
Interior: They are fairly different, but I'm fine with both; not in love with either. Tesla seems a little too minimal; Polestar seemed to have a few too many surfaces and textures. I like that Polestar offers cloth seats. There was plenty of room in the front of the Polestar (I'm not a big guy, so that may not mean much) but the back (especially the middle with that drivetrain tunnel!) felt notably more cramped than the 3. Some numbers online make the rear seat spaces look reasonably close, but when I put the front seat where my wife likes it, it was a squeeze to get in to the back of the Polestar. Thigh support was a little better back there than in the 3, but still not great. As for visibility, looking rearward in the 3 isn't great, but the Polestar was a little worse.
Utility: With the seats up and the cargo cover on, the trunk space is pretty comparable. But fold all or part of the rear seats down, and the Polestar has that nice powered hatch (plenty of ways to open it, including the "kick" motion) plus a couple of nice little net/strap/tiedown/divider niceties that definitely make me prefer the Polestar. (The 3's frunk is bigger; though still not big enough to really matter - not like my wife's X frunk, which is really handy!). Plus Polestar offers a hitch that can tow up to 2,000lbs. I don't tow, but sure would like to be able to put a bike rack on the back.
Tech/Infotainment: Hmm. There's an awful lot of stuff that is kind-of similar, but laid out differently, so which one prefers may largely come down to what they are used to (which is part of why I didn't spend a lot of time focusing on this...I could be much more helpful if I had a Polestar for, say, a week). The Polestar center display is smaller and lower-resolution...but it's got a really nice binnacle display too. It runs Android and while there aren't a lot of available apps yet, Polestar is supposedly working on adding a lot more. Their 360-display and ability to select the view of any camera is really nice. TACC seemed just fine, but lanekeeping had just enough trouble that I turned it off almost right away. Tesla has a lot of games, but I've never played them and I doubt (? though I'd love to hear if anyone has a contrary opinion) that even those that do would really factor it in to a car buying decision.
Acceleration: Yeah, the numbers are clear, a Polestar 2 is going to get killed by my M3P. And indeed, the Polestar is slower. But you know what? The difference didn't feel nearly as large as I was suspecting, especially when doing a rolling start at higher speeds. Maybe the Polestar ramps up quicker? You do have to press the Polestar's accelerator further to get the acceleration, which I wasn't fond of, but again maybe it's just a case of what one is used to. I've had the M3P for 3 years and LOVE the acceleration; before that I had an S P90DL for 2 years, and before that a Roadster for 7 years...but I really think I could live with the Polestar's acceleration. (Of course I'd still choose Tesla's if all else is equal!)
Steering/handling: I love the M3P's steering feel and was somewhat disappointed by this Polestar without the Performance pack. It had a great turning radius, and was perfectly competent and responsive, but it just felt a little numb. I sure wish my local dealer had one with a Performance Pack for test drives. Note that my drive included highway, industrial and residential areas, but unfortunately no windy country backroads to really test it out.
Ride and noise: The Polestar is definitely quieter and rides more comfortably. But I drove them back-to-back so it was easy to notice the difference...I don't think the difference on either count is really enormous.
Range and efficiency: The two cars have similar-sized packs, but the Tesla is more efficient and so it goes farther, charges faster, costs less to operate, etc. InsideEVs got 226 miles from the Polestar in their 70mph test, and 290 miles from a 2019 Tesla Model 3 AWD (not a performance; but I have 18" wheels on mine so probably not far off). For day-to-day driving around town either is probably fine; but on road trips you will notice the difference.
Charging: Yeah, there are way too many threads about CCS/Supercharger pros and cons already, so I'll skip this. Home charging speeds sound pretty similar.
Sales and service: I didn't dig in to details, but it sounds like Polestar wants to follow Tesla's lead on this (well, mostly - the place I visited was a dealership, rather than a factory-owned store). Order online, get it delivered to your house, and while they build out service centers, you can get mobile service. A great story and it may work well, but it will likely take them a while to build it out. Then again, given Tesla's service decline over the past few years, maybe Polestar doesn't have to do much to surpass them.
My conclusion: The Polestar is a really nice car, and I'd love to have the nice styling and especially the extra utility noted above. The little bit of extra quiet and comfort would be nice too; I am sure I would enjoy the car if I had one. But it wouldn't be any cheaper to buy or own (especially with the Performance Pack for steering/handling), and I don't think those small extras are enough to warrant giving up the 3's acceleration, range/efficiency, roomier back seat and access to the Supercharger network.