The x-arcade buttons are horizontal... they have generic microswitches i.e. NOT cherry (not that I currently can tell the difference)...
I think they are a little shorter than happs horizontals. They are about 1inch of thread space, so they'll fit nicely on a 3/4" thick control panel, but I'm I little concerned that it would be tough clearance wise (without routeing) with lexan ontop of the 3/4" ... (plug you can see a blurry shot or two of the buttons/joys on my halfassed photogallery/project page
HEREThese are kinda my practice buttons too, till I get around to getting fun multicolored ones and player 1 /2stick figure buttons (they are all black from x-arcade) . I may go with 5/8" mdf on my control panel only in order to accomodate a little more room.
Please be advised that Happs has a 25 dollar minimum order, so it can be a pain in the ass if you just want to order a handful of buttons or just a trackball plate, etc...
BTW I'd only do the sidewinder gamepad/other usb contorller hack if you are really tight on budget. I think it makes sense to save the headache and invest in a ipac now and just move it to your bigger, non prototype cabinet... On my cabinet, I'm using 2x sidewinder gameport style for now, but eventually I'll cut over to an ipac. But for like 14 bucks after shipping and an investment in time to hack them, I couldn't beat the sidewinders for a poor man's interface (well there's always a keyboard hack, but that's a different holy war in itself

Good luck...
rampy