Empire or First Order?

I'm assuming you are new to the game, I'd recommend farming at least a full team of each faction, rebel (Phoenix would be ideal for starter, used many places), sith, empire, resistance, FO, scoundrel, droid...

That being said, I'd start with Phoenix/rebels (devs love rebels, everything here is around rebels) and FO/resistance (new movie).

Also get scoundrel asap, for credit heist.

Empire:

You get vader shards automatically as you progress. You only need to buy a few to get him to 5*. His ship is great and he also has his uses in raids (with zeta).
RG can be farmed right away.
Tarkin is given to you at 4* as reward for ships intro challenges and is the best admiral until Thrawn and his ship will be available to you.
EP will unlock at 5* using a team of 5 rebels (many of which you will probably farm anyway — Ezra, ST Han, Lando, Biggs and possibly Ackbar and Wedge).
TFP is an easy farm in cantina and will also do well in both raids as well as fleet arena.

So, yes, empire will be both easy to get to 5* for R2 and can serve you well.

First order will unlock BB8, but they are a much longer farm and they require quite a few zetas to shine. However, FOO and Phasma are good in pit raids (Teebo lead teams without CLS), FOTP has a nice ship and Kylo still has his uses even if he's a bit outdated.

Of these two, I would choose empire (but yes, FO received a rework once, maybe they will again).

Kylo can still solo P1 of the Heroic AAT raid, and it prettty solid in all aspects of the game, why do you say outdated?

That being said, i did FO focus at first, but Empire can lead you to R2 which you need for Capitan Luke Skywalker. Plus, Empire teams are just fun.