FOTRSU: Super candy eye, great gameplay, lot of new ships, sounds, graphics, etc. But....It is a beta version, not finished yet. You can play it without many problems.
RSRDC: it is only a campaign to add to TMO , GFO or RFB. Historically accurate (as real as gets), but heavily scripted. After many patrols, you can know where are the convoys, warships, etc....This could be boring after many sessions.
Both are great.
Fitzcarraldo