I opened up a year old thread! Oh Noes!
Honestly though, this comes up in a google search earlier than any other link about this, so I figured this would be helpful for those of you who are debating between a prophet 5 and a prophet '08. Being an owner of both a Prophet 5 rev. 3.2 and a Prophet '08, I can tell you that there is a difference, but it isn't worlds apart as you might think.
If you MUST have the vintage P5 sound with all its quirks, then do yourself a favor and get a vintage p5. But also keep in mind there will be some maintenance involved with that decision. Curtis chips are expensive and difficult to get a hold of nowadays (unless you have a rev. 2, then you're really screwed with the SSMs). Power filter caps will need to be replaced (still available on mouser to do yourself, as they are screw-in for rev. 3.3), key bushings may need replacement, LEDs and switches may be bad, pots might be worn out, etc. It will be an uphill thing with your Prophet 5. If you drop 2+ G's on a P5, expect to drop at least another $200-400 on getting it going properly ($600+ if you aren't a do-it yourselfer), even on a good condition one. It won't be cheap, but there is quite a bit of satisfaction owning a vintage piece of gear. Just the fact that there are a limited number of good working vintage synths in the world makes it that much more special that you'll own one that is functional. However, the headache of dealing with tuning instabilities, memory glitches among other fun stuff (pot mux glitches) may or may not be worth it. I've had a few problems even in the very short time I've owned my P5 (6 years). And you'll be hard pressed to take it out of the studio unless you're Radiohead and can afford to buy a few of them.
The Prophet 08 on the other hand is rock solid, doesn't drift out of tune (unless you want it to), has many more features, and can pretty much replicate anything the P5 could ever do, and then some. The Mod source/destinations are enough to put a modular synth to shame (I know because I own one of those as well). And there are no worries that it will die and never come back, since Dave Smith Instruments is still active in the game and his support is top notch. I stress this, TOP NOTCH! They will bend over backward to help you with anything. I admit that the P08 lacks SOME of the guts/balls/growl or whatever you wanna call it that the P5 has, specifically in the low-end department. However, it seems to be a lot more friendly with the recording process. I find I have to do minor EQ with the P5 on low end and mid range whereas the P08 never has anything on it other than some reverb or delay on it in my recordings.
The one place where the P5 wins hands down is with Unison. I saw a comment on here that P8 unison sounds like the unison on a Juno 106. That couldn't have described it any better. The individual oscillators do not drift/fluctuate in a way that is pleasing to the ear in mono, even with the "Slop" turned up all the way. They seem to "slop" identically (hopefully they will fix this with future updates). It lacks some of the "realness" due to the missing pitch drift that the VCOs have on the Prophet 5. Depending on what you are looking for, this could be good or bad. If you already own a monosynth though, you have nothing to worry about. Even my lowly SH-101 does a better job of bass lines than the P5. If you have the bank to get a hold of a good monosynth for bass, do it.
In the end, if I could have only one of the two, I'd take the P08 over the P5. For me, it isn't a financial choice, but a recording/sonic choice. I end up using the P08 over the P5 most of the time because it cuts through the mix the way I want it to. It excels in synth pads and swells with 8 voices.