'03 thru '05's?
 

Is there any major differeces in body styles or performances between the
'03's and '05's?

No. but from an 03 to an 06, there are a little bit more significant upgrades, nothing big though.

05 6MT have the rev-up motor i believe, 05 has new interior design, 05 has option of 19" wheels

huge difference.......MUCH nicer interior, and the rev up engine........that classifies as a pretty big difference in my book

I have a dilemma. I have twin girls and currently own a Volvo S40 T5 and its 4 door but very small.
My wife drive a Dodge Caravan. I wanna trade in My S40 for a G coupe.
The twins are now just big enough to start using booster seats.
I've read through here and seems everyone is saying there is a
good amount of backseat room just that power seats take forever to lift.
What do you guys think keep the Volvo or should I get the G coupe I always wanted??

get the G... the G even has mounting brackets for the booster seats... theres a good amount of room in the back.. it only sucks for tall people cause they hit their heads on the back window