I did, tbh... he was THAT ...
I keep saying they're different players and persons... they're the same in that when you watch them play it looks like they're playing in Easy level while everyone else look like they're playing on Very Hard. Just otherwordly skill.
Diego, IMO, had much better control of the ball. Both on passing, free kicks and just flat out magic what he did with it (check youtubes, even when he was a kid, just mesmerizing). He was also more of a midfielder-forward (typical Argentina 'enganche').
Messi has the low-center-of-gravity dribbling goodness and, you have to hand it to him, he has improved his passing and shooting skill over time (still not Diego-quality, he was nailing free-kicks back in '79). But he's more of an actual forward and goalscorer.
As people mention Xavi and Iniesta now when it comes to Messi, back when Diego was playing, he basically 'made' Careca who he was. Careca would then go play for Brazil and do jack .
Personality wise, it's much easier to love Messi than Diego (unless you're a Napoli fan). Non-controversial all-around nice kid vs controversial, party-animal, drug-fiend Diego. Then again, some people loved the rebellious nature of Diego, and in a way was also the kind of at ude that made him excel against all odds (in Napoli, Argentina, etc).