Further to previous answers, I started a similar thread a few weeks ago - you may want to take a peek.
http://www.subsim.com/radioroom/showthread.php?t=171797
I think the upshot is to try both (with GWX and OM), if you like both then all well and good, if you don't then the worst is you're a couple of [insert your currency here] down - flog it onwards, no loss!
You can read all the forum in the world on the two games but only you can decide if you like it.