Recently it's been the v-cut more often than not. The main exceptions are when I have some sort of torpedo style cap or a box pressed cap that I don't think will lend itself to the v-cut very well. Then it's a guillotine.

I haven't had any big ring gauge cigars yet, but I have a punch cutter on one of my lighters that I've used from time to time that I'd probably use on a large RG cigar.