The Rollei 6008 and 6003 are really good models to get into. Both versions can use the grip, which makes hand held shots much easier. There was a 6001 version that took the grip, though the only metering on a 6001 is for TTL flash control. The 6006 is a model that is tougher to swap other parts, like backs, and has some other hindrances not found in a 6003 or 6008.
One issue buying a used Rollei 6000 series is that the battery might be dead or near dead. This is one expense to consider, since the recharge lifespan is limited after several years. Definitely should not get any of these without the charger, though buying a battery is no big deal. Outside of an old battery, these are very strong cameras and should hold up quite well to any uses.
Just going by the lenses, I would go more towards Rollei than Bronica. The Bronica gear is not bad at all, just that the Zeiss and Schneider lens choices on the Rollei are a different realm. The Rollei is also a bit quieter than the Bronica, if that makes any difference.
The hand grip is what mostly impressed me on the 6008i. I prefer shooting hand held (except for 4x5), and I found this to be the best medium format SLR I have ever used that way (my opinion). Anyway, best of luck whichever system you choose.
Ciao!
Gordon