Depends on your tastes.
The RH450 has a tone that is similar to a 'tube amp through a sealed 8x10'...eg not huge thundering lows, more of a kick in the low mids.
It won't have the really sparkly highs, but if you want that, the RH750 will give you that.
To this day, I've tried all sorts of micro's, and I still like the RH450/750.
The DI is absolutely superb. The tube tone is good, but very good at low levels. Depends what grind you like. The presets work well, the tuner is fantastic, and despite them limiting the watttage on the power module, (remember that there is no proof that its a low powered power module in there...more than meets the eye) you can crank it all the way and it won't flinch....at all.
I compared the 450 to a few 500W amps at the time and it WAS a louder amp. However they achieve that, it works.
Markbass amps tend to clip quite horribly when they hit their limits. They sound good, but they've lost their way since the LM3 and F500....no idea why they have so many different amps that aren't really as good as the LM3/F500.