I only own 7th Voyage, and so can't speak on Otherworld Skirmish, but you are correct in saying they are essentially the same system, just tweaked for the settings they are made to cover. I can't think you'd be disappointed in 7th Voyage; the rulebook reads well, is nicely laid out, and is full of inspiring photos. It even has a nice PDF supplement if you'd like to cover Odysseus' journey or the Labours of Hercules. If you decide to go that far, I'd also highly recommend the cards and tokens you can buy from Crooked Dice: they aren't that expensive and are beautifully done.
The truth is, all the above can probably be said about Otherworld Skirmish as well (though I don't think it has a supplement. Yet?). Again, they are worked up from the same design philosophy, which can actually be traced back to GW's old Lord of the Rings Strategy Battle Game. It's a robust system and can work in almost any genre. It sounds like what you have to decide--if decide you must (you could always buy both!

)--is which "atmosphere" you'd prefer for your games. I agree there is something very attractive about gaming Greek mythology: it is a little off the beaten track and so is a refreshing change from the usual fare, and of course has all the glories of Ancient Greece to tap into. And Harryhausen. The Arabian setting has some appeal also, though I don't game that one myself. But then, there is a strong nostalgia factor about old school dungeon bashing as well, and some cracking miniatures from countless manufacturers--Otherworld for starters--to tempt your painting skills.
It's a tough decision, no question. My guess is you'd be pleased with both.
Good luck with your deciding, and keep us posted on what you choose!