flat or straight, long or short, it is all personal preference, I have big fat in the palm hands so I prefer a long arched trigger and flat mainspring housing.... to me the arched mainspring seems to make the pistol point wierd.... and the curved trigger feels good.
for people with smaller hands and not so thick palms they would probably prefer a short trigger... and maybe an arched mainspring housing....
it is all personal preference and only experience is going to tell you what your going to like and what fits your hand, I suggest you try them all.... the parts and cheap and easily fitted. (just go slow and use water stones to fit the trigger)
as an aside the ONLY people I have ever seen use a flat fronted trigger has been on competition guns with trigger pulls measured in the oz's.... and then for the close in stuff I think they "slap" the trigger alot instead of squeezing.