I looked at some Rockwells back in 2002 when I was looking, and agree that the closest comparison to a TB20/21 would be a Commander.
I can't remember which Commander model was turbocharged; nearly all those I looked at were turbocharged, and the TB21 was then the real comparison. But the TB21, with +1000fpm at FL150 and a 25k ceiling, makes little sense in Europe without a full IR and a significant airways touring requirement, given the poorer MPG at lower levels, and the relatively small chance of any turbo engine making TBO which results in the engine fund being about £10-15/hr bigger.
However, the feedback I got (pre-buy) from general enquiries around maintenance shops was that Rockwells had many parts availability issues, grounding many planes for 3-6 months at a time, whereas Socata parts are usually very easy.