We've been to both quite a bit. From our experience, XClub has generally been a younger 'party' crowd (25-40s mostly), OZone tends to be an older 'laid back' crowd (35-55+). Those were Saturday nights that we've experienced.
We generally agree that we prefer the atmosphere of OZone (more laid back), but like the eye-candy (and sometimes music) at XClub more. (We also definitely prefer the no-fuss locker-access at OZone too!)
If you can, try them both! 
NYX Lounge and Oasis Aqualounge are nice too.
I've never been X club I've heard not fantastic things
We frequent ozone, the music can be hitting Miss lately play rooms are clean and comfortable again the bonus about the lockers. The age range is about 35 to 55 + very laid-back.
Nyx is also nice very similar to Ozone little further to the west of the GTA
Oasis at $95 a couple is outrageous in my opinion however it does cater to those looking for that kind of atmosphere and I can respect that
Oasis Aqualounge is only $95 (Couples admission) on Saturdays. Saturday is the most expensive day of the week @ Oasis. It's also open from 11AM to 3AM and you can leave and come back. Plus there's no annual membership fee like there is at some other clubs.
Considering that some Saturday Super Parties @ O Zone & X Club are $60 it doesn't seem that out of line.
We have only been to Ozone and enjoyed it. There are lots of play rooms in the back and many friendly sexy people.
The only draw back is the music is so loud it can be hard to chat with people. So it's a great place to go with friends, but if you are meeting a new couple and want to chat and get to know them, make sure you go for dinner somewhere before hitting Ozone.
The music is even louder at X Club than at O Zone. Another benefit of Oasis Aqualounge is that you don't have to try to talk over the music.
I won't use the word outrageous for Oasis pricing. They are downtown - that can't be cheap. For anyone who doesn't want to drive, they have a great location. They have a pool which is conversation central and gets quite packed. Add in a sauna and hot tub, no other club has that. However, these things run throughout the year and require maintenance and heating. The other places just need to park a dance floor in the middle of nowhere, so their costs are trivial in comparison.
We're definitely fans of X-Club. Beautiful club. Very modern. Amazing playrooms. Always packed, even in the summer. The dance floor is always jumping.
Tried Ozone several times last summer. Yes summer is slower. But it was pretty quiet (no issues finding a table). Quiet in front meant especially quiet in the back 
Sad because they too have some really cool play spaces.
I will say, very nice and friendly staff at both.
Each club has their own appeal and it unfortunately looks like you can scratch X off the GTA roster. Oasis is pricey but has a unique offering next to the other clubs. Attend when we're hosting and you can take advantage of the $50 group rate we get. If that's not appealing we don't know what is ! We frequent Oasis & OZone and in the summer we try to get to SandyBottoms a few times. The great thing about this part of the world is that we have LOTS of choices.
Mr OFL
Membership at The O Zone is included in your first visit, then you can take advantage of the discounts (and no yearly membership fee to renew). Reg. admission on Fri is $40/couple (or $20/cpl before 10pm), and Sat is $60/couple (or $40/cpl before 10pm). Frequent Flyers can purchase a Platinum Membership ($199/2 months) and there is often a Platinum membership special for half price (watch the site for dates for that). There's also a monthly Zodiac party on a Sat. night combined with another theme, and anyone who's celebrating a b'day can flash their ID to confirm they fall in that Zodiac window for a FREE couples pass. So if the price is keeping you away, look for the deals The O Zone offers. 
