Jem ship isn't really different enough from the Fleet Defiant to warrant the cost. It has a slightly better boff layout if you are not using a torp or dbb, but if you are the Fleet Defiants boff layout can be useful too. Jem ship has worse console layout then defiant, 1 sci console and 4 eng, vs 2 sci and 3 eng on defiant. Jem ship only gets 1.5k more hull and 3 more base turn which isnt all that noticeble of a difference against the Defiants. Defiant has option to use cloak console which is great in pvp, not so good in pve.
So overall Defiant is slightly better damage dealer while jem ship is slightly more manuverable and hull tanky.
I take out jem ships regularly and get killed by them regularly in my Fleet Defiant, they are both pretty well matched.
If that was the case, they wouldn't command the price they do.
Jem'hadar bugs do everything as well as or better than a Fl. Tactical. Everything. The only thing the tactical has going for it is the cloak, which eats a console.
You can even use tactical conn officers to free up the extra ensign slot to basically match the defiant in tac powers. If you do the same thing in a defiant you end up having to take a mine launcher or DBB to utilize the extra ENS slot.