building my engineer up an it will be either one of the 2 so i just want input on pros and cons of the 2 ships from everyone what are some good builds for each who would will win a battle with an identical setup

The Excelscior edges out the Assault Cruiser in terms of damage capability due to it having a LT CMDR Tac BO giving you the option of running BO3, HY3, FAW3...etc. It also has one of the best turn rates of the Cruiser family.

The Assault Cruiser will put out comparable damage, but it is more "Tank'ish" with a more traditional Engineering BO and Console setup giving you more support tools. This also lets you run 1-2 DEM copies without sacrificing too much healing (as you would with the Excelscior).

NohMalkiri is right, the Excelsior does outdo the Assault Cruiser in DPS, however, not by a whole lot. I decided to go with the Assault Cruiser for 2 reasons.
1. Money. The Assault Cruiser is free, while the Excelsior is going to cost you money.
2. I wanted to be in the middle. I wanted to tank, but put about a lot of DPS. I have seen the Excelsior tank very well in PvP, as in I couldn't kill it with my Tac. (This is my Eng toon)

So I am running the Assault Cruiser, and my DPS is very good. Here is my build, and it is actually really good in PvP,(Meaning you will never die one on one) and PvE is a joke. If you have a ship set up for PvP, you will win in PvE, but the opposite is not true.

Tac Boffs
TT 1
APB 1
TT 1

Eng Boffs
Cmd.

EPtE I
RSP I
EPtS III
DEM III

Lt. Cmd.
EPtA I
RSP I
EPtS III

Sci Boffs
Polarize Hull I
Hazard Emitters II

I am using the full aegis set now, but plan to use the borg set, with the aegis shields.

Eng Consoles:
+7 to weapons
EPS +105% power transfer rate
Nutronium Alloy
+35 to shields

Sci Consoles:
Halon System
Borg console

Tac Consoles:
3 + 26 to Beam weapons.

I run 8 Phaser beam arrays.

Hope this helps.