I don't think you can go wrong with either brand, just pick the one with the tools you want/need.
Saws and files are great on both of them with Leatherman having a slight edge here. Scissors are better on Victorinox. Leatherman has more options with one-handed opening knives, although Victorinox one-handed opening knives are smoother to operate.
There are a lot of differences concerning screwdrivers. The Leaterman Wave often gets mentioned as the best/most balanced tool of the Leatherman line-up but I personally don't like the screwdrivers on it. The bits it comes with are often too small or too big and don't have much reach. The Rev/Wingman/Sidekick screwdrivers are more useful to me.
Victorinox has a long Philips on some models, even 2 on the Hercules, which are great because of the reach. The can-opener and bottle-opener screwdrivers fit on a lot of things, but have limited reach and the can-opener one has a sharpened edge facing the screw which might render it completely useless.
Guarantee wise they seem to stand by their product. Just keep the box/receipt.