My buddy chris and I have come full circle, from mg300s (running into a neumann 48 volt ps > sony d7) to mbho 603a (first into ps2>adc20, later into oade m118>apogee ad500) to schoeps mk4/kc5/cmc6>oade m118>ad500/modsbm1, and now back to mg300>tmod ua5.
We started down the upgrade path due to some distortion issues with the gefell>phantom>d7 rig. If we'd invested in a pair of line transformers (or merely added the ad20) instead, we'd probably never have upgraded.
Bottom line, definitely prefer the mg300 sound to the mbho 603/ka200 sound (personal preference), and am still having trouble picking a favorite between the schoeps/gefel setups, but ymmv.
Listen to some recordings from each and make up your own mind. Both are real nice, but very different -- IMHO, the gefels have a nice warm and rich sound, but lack a bit of detail. MBHO has the crispness and detail but lacks some warmth. Also, I found that running the mbho's fob, they sounded "farther back" than the schoeps/etc. running around us.