cyanjaguar-
I know I'm copping out but I like them both. I rarely track vocals because I play guitar and synths in a four-piece instrumental band. However, I have used the SRV-330 for vocals in another studio and I think it sounds so dense and realistic. I love plate reverbs and this one has very good ones. The Lexicon is easier to edit on and has great reverbs too but I never use it for vocals, so I can't give you a definitive answer to which is better.
I love the MPX-500 and I use it for so much more than just the reverbs. The rotor-speaker and chorusing are a big step up for me because I'm using many older synthesizers with really dated effects. Like the Korg 0/1W. Great keyboard but really bland, lifeless effects.
If I wasn't on a spending freeze I'd be tempted to buy the Roland. I saw the SRV-330 at Guitar Center and they were selling them for $300 US. It was tempting not to just put it on the credit card. I really don't need another reverb.
I use a small amount of reverb. I love 70's recordings that were almost dry or used ambient spaces instead of processors. I often argue with my recording partners during mixing. My mixes lean towards the dry side.