So we're all clear, an up-locking reel seat is just that - it secures the reel onto the rod towards the tip end of the rod. The fixed hood is located inside the end of the cork grip & the moving, or locking, ring / hood secures the reel foot closest to the butt end of the rod.
If you asked a rod designer at a large rod company today for the technical reason why they use up locking seats more, they would probably tell you it balances the rod out better. As fly rods get lighter in overall weight, they want the weight of the reel closer to your hand. I suspect the real reason they use this style so much is more because of fashion. Hey, nothing wrong with that.
Mostly, I tell clients who ask that it really doesn't matter. A reel seat is normally 3 1/2" to 4" long. Moving the reel up or down an inch along the reel seat makes very little difference. So, pick the style you like most.
There are a few exceptions though.
On very light weight & very fast action rods, I suggest an up lock because on these rods even just the weight from the smallest reels will be felt. Putting that weight on the very end of a quick, light rod may make it feel unbalanced.
On heavier rods, such as big 8 weights for large bass that might have a more moderate action for example, I suggest a down locking seat. The larger & heavier the rod, the more I recommend a down locking seat in general.
I always suggest a down locking seat on bamboo rods, especially the mid-sized rods made for trout. I do this because it's traditional on bamboo rods & because it does tend to balance these rods out a little better, however either one of the two styles is acceptable. On really short & light cane rods it doesn't seem to matter as much.
So unless you've got a rod that is in one of the extremes in weight & size, or a bamboo rod where you want a more traditional look, that I say choose whichever style of reel seat that pleases you the most. Either way, a quality reel seat is a thing of beauty no matter which way you go, up or down!