Phillips. Part of it is that I don't find using Allen wrenches to be pleasurable, but mostly it's aesthetics. The other slight factor is that way, way back, my family had several Phillips screwdrivers all around the house, but no Allen (or at least one that wasn't hidden in some corner of the garage), so I would have really needed to keep track of and stay on top of not misplacing my own, which would have been a pain. Another angle is I think that I kind of equated Allen with Grindking, which were awful, although from this vantage point the Allen aspect was probably their most legitimate aspect. Historically, I know that the Grindking kingpin modification on Indys was popular and absolutely had its merits functionally, but if I don't really like Allen hardware visually, I find an Allen kingpin to look especially offensive.
The psychological aspects at play here in the aesthetics preference are interesting and kind of hard to fully parse. For me, the Allen hexagon basically registers as a hole, especially standing on a board, and my mind somehow doesn't jive with the holes (although I'm otherwise not among those with that general hole phobia). I guess Phillips visually seem more solid. Some might see Allen as looking cleaner, but to me a six-sided hole is a lot busier then a small "x".
As an aside, for the past several years I've maintained two (non-repro) late-'80s/early-early '90s set-ups that I fully skate using era-appropriate NOS Truss head (Phillips) hardware, and when I go back to my contemporary board, I really miss feeling the hardware – I like the physical sensation when skating. I haven't tried using Truss head on a popsicle, but I wonder if my mind will respond negatively to the visual discrepancy of "mixing eras" – I don't think so, but maybe the proportion will seem off on a smaller, in several ways more streamlined set up? There's this weird balance thing at play in all of this. I conceptually don't have an issue with mixing eras, and I actually fundamentally welcome moving beyond any status quos, but, as an example, I don't respond to seeing shaped/directional/repro decks with a non-conical wheel. I don't really like that I have an issue with this, but I haven't been able to ever like an example of this, despite trying to be looser about it. I'm also currently in a place though where I just prefer conical wheels in general, UNLESS a crook of any type is being done on a ledge or curb, in which case both functionally and visually I want to see a rounded, Spitfire classic shape.
I've been skating conical fulls for a long time now, and I exclusively do (a lot of) nosegrinds (from all four stances – just to be clear, this isn't from a hellride perspective) and no crooked grinds. It works hand-in-hand though because I prefer nosegrinds to crooks; on a small level I think being on top of the grind feels more legitimate and fleshed-out, otherwise I would rather opt to do a noseslide.