Select one that can be mounted either way and decide for yourself. That's the best way to decide--rod in hand, trying the feel with intended reel(s), and with your prefered grip shape and length, and your chosen guide-spacing in place. If your builder is close by, it is easy to shim fit the reel seat temporarily for a try-session. For that matter--although your builder's advice might come first--anybody can glue on a reel seat, so if it were fit for you, the rod could then be provided complete except for gluing it on the reel seat in the position you decide on.

Oh, well said by SCR, and didn't mean to give any other idea. If you don't know how to fit select and fit components, you would need more detail from a vendor like SCR and/or your custom builder. The length of a reel seat, whether its upper hood is concealed by the cork if UL (as is the current fashion)--all affect feel, and all can be customized to user preference by a builder. All can be set up in a temporary try fit. Only the user can know which fits and feels best in hand. A hobby builder could also order two different seats--one UL and one DL--to try. Whichever isn't used will be just right for some future build anyway. But choosing a stock component and having it mounted in a stock position may be all the "customizing" you need--again with the advice or try options of your builder.