keep the phoenix!
also, since they're both in game, try em out first. TEST has several members with either and probably some with both.
the valkyrie is a very limited ship; it has one purpose, and that's moving troops around. More than that, it's specifically designed for planetary engagements and isnt going to be anywhere near as useful in ship-to-ship combat.
the phoenix, on the other hand, has several uses: it's first and foremost a luxury transport, it doubles as a cargo/smuggling ship, it's not going to suck at combat and comes with both a rover and a support fighter, so landing it and using it as a mobile ops center is a valid strategy, and, if you're not picky, you can probably cram a similar amount of people in as you can the valkyrie, although deploying wont be anywhere near as safe and they'll have to bring their own air.