I'm with maddie on this one. Straddle presses are easier, but if you can pull off a straight-arm pike press, it tends to be a head-turner.
The difference between a straddle press and a pike press is much like the difference between an average handstand and a perfect straight handstand (ie Sasha Artemev on p-bars). It's not big and flashy, and will probably not make a huge difference score wise, but it makes all the coaches stop and think "wow, this kid's good."
As for my preference to do, I like pike presses on floor and p-bars; they just feel so much more satisfying when you hit them (and, at least on floor, they're worth more)
On rings it depends on the situation; if I'm being lazy and just getting up to handstand to work some other skill (giants, dismounts, etc) I'll usually do a straddle press, but if I'm having a good day and/or focusing on strength and/or have an audience that includes any girls my age, I'll do either a pike press or (on a really good day) a straight body press.