I'm finding that I like longer tips on either of these types of setups, whenever I can make them work. Shorter tips make for better presentation tools, IMO, but if you can't get the cast to the lie, presentation doesn't mean much, so I usually make the compromise. With compact heads and tips shorter than 10 ft. or so, and especially on longer rods, I often struggle to keep my setup anchored when forming the D loop. That's probably the result of being a bit too aggressive on the backswing (read that as "my casting faults") instead of trusting the rod action to generate the speed and power. Anyway, using a longer tip and adjusting the grain weight down for the additional length, as opposed to using a shorter, faster-sinking tip, seems to help with that.

As for presentation, most takes seem to happen almost immediately after the fly starts swinging, and a big part of why probably has a lot to do with how the fly moves after it comes under tension. I think shorter tips probably cause the fly to rise away from the fish more quickly as it swims (maybe it mimics prey trying to escape and triggers a bite reaction). Whatever the case, figuring this stuff out is part of the fun of Spey fishing.