the 20v 147 imo has got more of a kick than the 155,feels more lively.To which that is due to being cable driven.A few on ere and ff have said pretty much the same.(but its down to own preference) A simple chip and exhaust/ind kit/good air flow will push you to around the 160 mark.With the 155 you can only remap it which is expensive in my eyes for an extra 8ish bhp.But if money aint an option you can achieve around 170bhp with a sports cat and exhaust on the 155.
Now the alfa,i cant say as i havnt had one.
had my 147 4 years now and i love it..when it works lol