As a former BB614 and current RBX775 owner, I can say they are very different basses.
I wouldn't expect to get Fieldy type sounds from the BB - it is a fairly traditional sounding P/J bass. The RBX is much more modern sounding and would suit metal a lot more than the BB. That said - the BB is indeed versatile and can do a more all-round job than the RBX.
If it were me though, i'd go for the BB. The RBX375s don't come in the best of colours (dark red, silver and [url="http://data.yamaha.jp/sdb/product/image/main/raw/r/rbx375_mastered_pearl_effect/D4B385814A3E4F39A6188B8AF02E9388_12073.jpg"]'jaundice yellow'[/url]) and look a bit cheap close up. A pewter BB615 should look pretty damned awesome in a metal band and will serve you well.