You know, on the one hand, I think to myself, "these heads are so different, it's hard to compare them", but then on the other hand, they both kinda aim for a modern take on a vintage (tube) tone, so I can see why there is an interest for this.
The short answer is, the SA200 sounds better, and the DB 750 gets louder.
Now, for the long answer, I should probably glean through some prior posts and piece together a more comprehensive response, but let me try to paraphrase.
The SA200 is a supremely musical sounding head. It can do a very traditional (and very good), full, warm "tubey" sound, but it can also get pretty quick, articulate and snappy (more of a modern tone), while still maintaining a pleasing girth. You can dial in a low end that is a bit on the big, round and slightly soft side but you can also keep the lows nicely tight and full. No matter what you do with it - and so far, no matter what cabs I play it through - it just sounds "right." And while I say that the DB 750 is louder, the 210+ watt SA200 (with the right cabs) has more than enough juice for even my loudest gigs (two SA410's with the SA200 get stupid loud). But the beauty is, the SA200 sounds great at any volume, which I find is somewhat unusual, especially in tube heads.
The DB 750 is potentially a more fickle beast. It has gobs of power, and the power amp section is truly top notch. It is full, warm and sweet (the power section). The frustrating thing with the preamp section on the DB 750 is that while it looks like the same layout as the DB 659, it certainly doesn't sound like one (and I prefer the 659, but that's the subject of another thread). But, at the right gain setting (generally pretty loud) and with the right cab (so far, the Berg NV215 and my custom Azola Super Wizzy's are my strong favorites with this head, though two NL-210's are really great, too), it just dominates: very full, very articulate, dynamic, aggressive, and just an overall visceral experience. I am not as fond of this amp at lower volumes, though it's certainly far more flexible in this regard than, say, a vintage SVT. The DB 659 into DB 750 power section actually gives the SA200 more of a run for its money.
Hope this helps, Tom.