We like both for different reasons.
Currently we are predominately NCL and we usualy find we can pick cruises where we know people(passengers and crew) that helps improve the experience. This will happen if you start to repeat ship/line a lot. on Price NCL is currenly winning except for these late deals where RCI and celebrity are edging ahead.
Entertainment can be hit/miss on both lines but the more you cruise the more the offerings become repetative anyway. We find on NCL we focus on dining and fit stuff round that unless there is a must see act, with a group 2-3 hours is fine, not so easy with trad dining.
With NCL I think you have to do the alternative dining to get the best out of what the ships have to offer so that ups the budget a bit $100pp more than covers it on a 10-12 day trip and has still been cheaper than a RCI equivilent.
You have to adapt to the freesyle dining so if you like traditional, same table, same wait staff, then that needs to be thought through carefully.
We like the size of the Jewel class NCL ships but one thing missing is a bar/area that tends to have good lounge entertainers, RCI do seem to this much better and the bigger RCI ships the ice rinks add an extra dimension to the offerings. Also no multi story atrium on the NCL ships.
What NCL does have is a much better set of perks once you get into suites, and there are some really good deals over the winter for the Villas, which is a completely different experience, on the Jade, we have shifted our normal late in the year cruise(some where warm) to a courtyard villa, med Jan to take advantage of these offers for the same money we would have spent anyway(inside/outside) on another cruise.
I would steer clear of early bookings these days unless there is a must have need or a great deal, there are going to be bargains galore late bookings. Look out for a deal in an AE or above on NCL. For the BIG ship experience RCI still have the edge with the voyager class.
One other thing is that ships can change a lot over a few months so if a ship goes down hill after you book you are stuck, late booking you can keep an eye on the upto date reviews.
If you are looking at 2010 why not pick one of the short cruises next year to give NCL a try and see if you like the ships.