The fundamental question between the two is do you want a diver or pilot watch as your next purchase? Once that question is answered, then I think your decision will be a little easier. At least you'll have a direction to go in.
Regarding which will become more valuable, I first have to comment that watches in general are a very poor investment. Fact is, unless it's a watch with a deep and wide cult following like Rolex enjoys, you're going to lose money when you sell it. Having said that, the Lemania 5100 is a fine movement and, additionally, is no longer being produced. When the last of these current watches are gone, that will be it. So, between the two, I'd say the Sinn will be worth more in resale value, but only to that small market of people that will appreciate the movement and its scarcity, i.e., WIS types. :-D