I would think it shouldn't be all that difficult to pull a 1 or 2 bedroom unit there for your dates. I would think that March and April would be a little tougher since the weather is reasonably nice then. Jan and Feb can be a bit on the cold side, May and June will be getting into summer time temps.
3 bedrooms are likely to be harder to come by as they lock-off into a 2 bedroom and full 1 bedroom unit. I can't imagine a lot of owners depositing the entire 3 bedroom unit. I know we plan to always lock ours off unless we need all 3 bedrooms for our personal usage. When we exchange, I plan to always lock it off and look for other Marriott resorts to exchange into using the Marriott to Marriott exchange advantage and lower exchange fee's.
Is it the best in Vegas? I guess that depends on what you want out of your timeshare resort. Grand Chateau has a great location and excellent rooms. The roof top bar provides one of the best outdoor views of the strip at this time. On the other hand, the convenience store is very small and high priced and the pool is laughable (very tiny). I haven't seen any cabana's around the roof top pool to provide any shade from the heat of the day either so, it's going to be hot, hot, hot up there with a tiny pool only 3 ft deep to try and cool off in. If you're a pool fanatic, then you definately won't consider this the best resort in Vegas. When we were last there, Grand Chateau owners were allowed to use the roof top pool at Polo Towers or the pool at Planet Hollywood just by showing their room keys.