I’ve got a 24x3 nimbus and a 29" nimbus.
Both are great.
Generally the 29-er is considered best for communting, but I use the 24x3 a lot for getting around on.
Do you want to commute on the roads or the pavements (sidewalk)?
The 29-er is good for the roads, having more presence and speed; on the pavement, particularly with the usual 5" cranks, it’s got disadvantages in that it’s more intimidating to pedestrians and less controllable.
A 24 x 3 is slower, but easier to control, good amongst pedestrians, ok for short bits on road, but not for continuous road riding.
If where you live is exceptionaly hilly, then a 24 x 3 will have the advantage- Sheffield, where I am is not idealy suited to the 29-er with 5" cranks, but few places are as hilly as Sheffield.
Another option is to fit 6" (150mm) cranks to the 29-er, getting better control and making hills easier- this is my favourite 29-er set-up.
A 29-er with 5" cranks is theoretically faster, however I did find that to have good control with it meant very regular rides; particularly if you’re on the roads- you have to be a good confident rider on the roads.
Whereas the 24 x 3 is a machine that you can jump on after a long lay off, and still be able to ride it well.
A 29-er with 6" cranks lies between the two in that respect, probably closer to the 24x 3 in terms of control.
There’re both great unis, you won’t go wrong with either of them 
If you get a 24 x 3, I recomend getting it with a 29-er frame switched in, this gives you the option of putting in a 29-er wheel later on.
Here’s links to some previous threads that may be of interest, doing a search or following links will turn up loads of info: -
http://www.unicyclist.com/forums/showthread.php?s=&threadid=34147 (discussion of pros/cons of longer cranks on big wheels)
http://www.unicyclist.com/forums/showthread.php?s=&threadid=29517&highlight=hills+AND+24+AND+29er (a thread on commuting on a 24 x 3 muni which elaborates on some of the points made above)
Lastly, to a large extent ‘good for tricks’ = ‘not particulalry good for commuting’ and ‘good for commuting’ = ‘not particulalry good for tricks’; so you’re going to have to come to a decision on that one.