Spaceships:
>can fly independently through space
>can carry lots of weaponry
>can transport entire armies, equipment, and anything else you may want to transport
>have captains
>comfy home for as many cute girls as you can fit
>long standing naval traditions to lend heritage
>more than enough comfy escape pods
Mechas:
<capability of long distance space travel limited severely
<amount of weaponry limited severely
<can't transport anything besides the pilot
<don't have captains
<cramped and generally piloted by one guy
<no traditions at all, literally untested tech
<ejection seat at best, not comfy at all