A point to take note of is that in most auto blogs, both cars are considered overpriced for what you get, esp. with the buzz about the upcoming GT-R and ZR1, and the fact the R8 can't match the straightline performance of the older 911 Turbo and Z06.
I tend to think the Roadster has more value in it for other reasons besides from the performance specs on paper, but many sports car fans don't agree. People keep bringing up comparison with the Elise and Exige, and say "save your $50k, it's just an Elise with an electric motor, the Elise is already very fuel efficient anyways."
In regards to it's 100k price, I've seen this comment on autobloggreen's firstdrive youtube video "98 grand for that pos?? the upcoming Corvette ZR1 will be the same price, I'd get a ZR1 long before this crap. plan on going any distance in a tesla? then i hope you can deal with no radio, no headlights, no air and no heat on, the you may break 200 miles with the wind at your back."
The funny thing is that this guy seems to have been following the Tesla Roadster from the start and he still makes this comment.
Sorry for bringing in so many negative things, but the final production version of the Roadster isn't out yet, so we really don't know much about how it is going to turn out. Doubtlessly people will first try to compare using the current temporary transmission version with 0-60 in 5.7, which IS too slow for your money, esp. if they consider 4.4 seconds in the R8 to be slow in the first place. For comparison sake, the 911T and Z06 get around 3.7 secs for 0-60. The old two speed roadster was probably capable of that in ideal conditions if I remember correctly, but we don't know enough to know if the final can get sub 4 second times in ideal conditions.
I think it's more appropriate to do this comparison when the final version is out, and I'm pretty sure the magazines will want to do comparisons too. Right now we still don't know enough about the final production roadster to do any meaningful full comparisons of performance.