I'm quickly growing my Imperial and Rebel collection, and now in the near future I must turn my attention to either Scum or Epic ships.
What do you like and why?
Go Scum or Go Epic?
Tough call...if you play competitive... Epic for the upgrades.
If you play casual with friends... Scum. For the variety.
And of course....
BOTH.
Because...X wing miniatures.
Go Epic and get a bigger Imperial Fleet!
I just play casual.
Are epic games really long?
A super huge part of me wants to tell you to do Epic, as the ships are fantastic. However!
Most Wanted and the StarViper will make your existing ships significantly better. In addition to being great Scum boxes in and of themselves.
On the other wrist, Epic is basically a whole other (awesome!) game in and of itself. So you can delay getting into it for a bit.
Having said all that- Four Aggressors is an amazing Epic list.
I just play casual.
Are epic games really long?
The first few times you play, yes. Past that you get the hang of the extra few steps and things speed up enormously.
Really that's the great thing about this game....you can buy ANYTHING and its useful and or fun.
.my girlfriend said she didn't want to buy me ships for presents because she didn't know wich ones I allready had or wich ones I was looking for.
Turns out...doesn't matter. Doubles..triples...something I never planned on being even one of...its ALL GOOD.
I say go both. 
(shameless plug) click link in signature.
All of it!!!
Because you never know what you will want to play!
I committed myself to play mainly imperial with a bit of rebel stuff when I first started (2 months ago). So the logical choice for me is to go for Imperial Epic ships, which I did. Scum is never a true faction anyway. Now I have 1 Slave-1, 1 Most Wanted, 2 IG-2000s, 1 Punishing one and 1 Mist Hunter on the boat™.
Go Scum. Some of the upgrades are handy for any fleet, and the flavour of the Scum ships is just a little different, so I think they make a nice mix.