I'd say ribs, as you want to do something that won't force you spending a lot of time at the end doing a LOT of prep.
People like their steaks in varying levels of doneness - that and having to do a final sear on 29 steaks would make it a little problematic from a timing standpoint.
When is the event? (if you say this weekend, then you really don't have enough time for how I do my ribs)
I normally cut them into half racks, then roll them in olive oil, then apply the rib rub and put them in the large freezer bags (1/2 rack per bag), then in the fridge to marinade in the rub for 24 hours (or at least overnight). Then a sous vide cook at 145F for 36 hours (removing the air from those bags via the water displacement method). Then you remove them from the bags, put them on a grill of at least 350F (you need the sugar in the BBQ sauce to caramelize to get that awesome stickiness). This last step, you're really just carmelizing the sauce on the ribs - they're already cooked - in fact, you need to be careful flipping them, as they're falling off the bone tender (in fact, sometimes, you'll find bones free themselves in the sous vide pouch)
Time on the grill to bake the sauce into super-sticky perfection - about 45 minutes.
I'm a fan of the Bull's-Eye Guinness BBQ sauce (if you can get it) - I know Walmart carries it in both the US and Canada.
Oh...and if you have a good liquid smoke that you like - a few drops in each bag, just before you put them into the bath produces that wonderful smoked flavour.
...and +1 for what Richard said - you need to use hot water for your bath - the reason that there's vessel limitations on the specs for your precision cooker is that you don't want to over-work the heater in the APC. I'd fill your vessel, let the APC bring it up to temperature, then put in your multiple bags of ribs.
If you're in the US, you can also get some very good deals on the LIPAVI sous vide racks, so you can maximize how much meat you can put into your bath while keeping them from moving around too much on you.
Oh...and, so you can make an informed decision on the level of doneness for your ribs, a phenomenal article on sous vide ribs over at serious eats!