DR3000 SLR5500 what's the difference?

There are, actually some significant differences between the DR3000 and SLR5500. Most notably the size, and support for MPT1327 site controllers. But there are some other, lesser known, differences in terms of specifications: (Green = Better; Yellow = Slightly better).

Of course, the SLR5500 is a made-for-purpose repeater whereas the DR3000 is essentially two (modified) mobiles.

More about the SLR5500 here.