It depends on the relative prices of the units and your budget. If the price of the DX2 with CD drive is in the ballpark I'd go for that.
The DX2 has better UI and more advanced cheat codes (although you can't load them from a disk?). It uses standard SMC format ROMs unlike the SF7 which won't be a problem if you dump all your own games directly to Game Doctor format, otherwise you'll need to invest some time learning to convert formats.
The SF7 is a good unit and if the price is significantly lower it's also a good buy.
I doubt there's a reason to consider a DX if you can get a DX2 or an SF7.