It depends on the goal. If you want to be tougher, you go Inv. If you want your damage output to be up in the "unholy frell" range, you go bio. I've got a DM/Inv brute that just doesn't die, but I would never bother going to check his DPS.
How expensive are my builds? I'd go with "standard expense for 5 purple sets and a properly considered Mids build." That said, I've run a LOT of Bio users. Some at lowbie levels, some in the 30s with nothing but basic IOs and a Perf Shifter set, some tweaked hard at 50. Bio is pretty dang solid the whole way. Will it ever be as "fark you, you can't kill me" as Inv? No, absolutely not.