I've been to both and each have their pros and cons. I was in Whistler for most of February 2013 and Japan for 3 weeks in January 2013.
Whistler is a great place to go if you want to board an enormous mountain (seriously, I was there for almost a month and definitely didn't see all of it) then party at night. The village is really fun and there are plenty of good pubs and places to eat.
We stayed over at Blackcomb and rented an apartment while we were there for a pretty good price. It was only a 100m walk to strap in and ride down to the Wizard (I think that's what it was) and off you go. Really convenient and cheap if you've got a crew to split costs.
The mountain's got a lot of varied terrain and excellent parks if you're a park rat. The snow was pretty piss poor when we were there, we only had a couple of big pow days but it was fun all the same.
For me though, the biggest downer of Whistler is that it's really fucking busy. I get the shits with lift lines and it was very rare that we'd wait less than 10 mins in line except to get on the gondy from the base of the hill.
Japan is quite different in a lot of ways. You're guaranteed to have some epic epic pow days. March would be a good time, Feb would be even better. The snow's just so fucking nice.
We were in Niseko, it's a great mountain also and you can access some pretty amazing side country through the gates. It's pretty damn big too, you can get some really long runs in if you want to go top to bottom and on a pow day you'll be smiling ear to ear the whole way down.
The people in Japan are without a doubt some of the friendliest you'll meet. I fucking love those guys. I had so many drunken nights where we ended up smashing saké with Japanese guys who spoke zero english while we spoke zero Japanese. Fucking brilliant times.
Japan does get a little mobbed by Aussies (I'm one myself), particularly Niseko. But if you're from the UK your likely used to seeing plenty of Australians and kicking yourselves for exporting all the hot women and keeping the pasty ones. It's not too bad but there are times when even I hang my head in shame at the behaviour of the 18-25ish year old crowd.
Errr what else... the food on the mountain in Japan is the shit. In fact, the food everywhere is the shit. Beers are pretty cheap and there's a bunch of good spots in Niseko to grab a feed and a drink.
Both Whistler and Japan seem to be on par in terms of cost so I wouldn't consider that a game changer.
That's about all I can think about man. If I were to go on another trip to Japan (I should be saying "when I go") and I had the missus with me I'd like to try out Nozawa Onsen but for a boy's trip I think I'd hit Rusutsu.
ETM is the guy in the know about Japan. He's driving a van around the joint making a general prick of himself now.
I don't think you can go wrong with either spot. I would head to Japan though, it's just a different experience to going boarding anywhere else. Particularly if it dumps. That's a special thing.