I own:
Peavey OG 5150
EVH 5150 III 50w v2
EVH 5150 III 100w Stealth
EVH 5150 III 100w EL34
I have extensively tried:
EVH 5150 III 50w Stealth
EVH 5150 III 50w EL34
EVH 5150 Iconic
As somebody with access to a TON of pre and post effects and who likes to think he knows how to use them, like pre-EQ's, boosts, overdrive pedals, parametric EQ's for loop or post processing, it's hard to beat the OG for its raw nature and how well it takes to being sculpted by pre and post EQ. You can do a lot with it.
The 100w Stealth and 100w EL34 feel like the OG but more "studio tweaked" like they come from the factory with a bass cut built into the input for tightness and honestly a better tonestack that sounds more balanced across the frequency spectrum than the OG 5150, but 5150 III's don't have quite as much fat, thudding low end as the OG 5150. The 5150 III can still get very low end heavy, I'm not saying it's thin at all, it's not, but it doesn't have quite the same wallop as the OG 5150. I think this makes the newer amps on their own sit in mixes easier and sound tighter and more articulate out of the box, and they're easier to EQ using only the amp's tonestack, but the other side of the coin is that they're not quite as fat sounding as the OG and their more filtered nature means you have less room to shape them however you want.
They're all great, but the OG feels more "raw" and therefore more malleable I guess you could say. BTW when I saw "raw" I don't mean "more aggressive" or that there's a different quality to the grain in the distortion, but raw as in it doesn't feel like it's already been EQ'd by a studio. Again that's not necessarily an inherent positive or negative, just a difference.
If I had a budget of $1,000, I'd get a new Iconic, no question, wouldn't even think twice about it. I haven't talked about it but I think the Iconic actually strikes a balance between the OG and newer 5150 III amps which makes the Iconic its own thing in that way. They're awesome amps and I'd recommend anybody interested in high gain to try them.