With either one, if the wheel slips it will slide forward on the drive side only and slam the wheel into the non-drive stay. I've seen this at the track with worn axle nuts and I've seen it on the old horizontal drop outs with worn or misadjusted skewers. People aren't strong enough to wrench the whole wheel out, only the drive side slips and in which case which version doesn't matter, the result is the wheel jammed into the frame. The horizontal style are easier to get a wheel in and out of and past a derailleur while the track are harder to get the wheel in and out of especially if it actually has a rear der. If track riding, get proper track ends, anything else, horizontal dropouts are easier.