From what I've read, when it's hot and you want an open boat, the partially open white water deck is still fine. Things don't get too hot in there supposedly, so I wouldn't be too worried about that. Where it might be an issue is if you want to cram a dog in the boat, and it might interfere if you want to recline for a bit with your legs up on the tubes....I'm not too sure. Hopefully someone else can chime in.
I think deck choice is mostly a question of cost, whitewater functionality and weight. With the whitewater deck you're looking at an extra $50 and 6.5oz (14.5oz vs 8oz) over the cruiser deck. In my opinion, the Cruiser deck wins when weight is important and the water isn't going beyond class III. You save money and weight, plus on fair weather trips you can ditch nearly all the weight. With the white water deck you've got a good portion of the weight permanently on there, and if you actually want to use it then you're going to be hiking with the pole set too, which is another thing to pack and it puts you near 6 lbs, which is actually about 9-10lbs with a paddle, PFD and gloves. That's quite a bit of weight and each decision adds up.
I'm sure the white water deck is way better though on rough seas, it's pretty easy to take on water with the cruiser deck because it's tough to get a good seal around your body. Not an issue for class II, and a good paddler can stay dry in class III, but beyond that you're going to get somewhat wet, which isn't good in the shoulder seasons.
I'd probably decide on the drysuit first, because if you get that then you could potentially get away with no spray deck and it would be the lightest setup of all if you can leave your rain gear at home. A drysuit + cruiser deck would be a pretty solid setup, so if you get the drysuit you wouldn't really need to go with the white water deck too unless you really evolve into a serious white water paddler.
With your inseam, you may find it best to omit the backrest in a Yak (Skurka style). I find the backrest doesn't do that much and omitting it will add some legroom + save ounces.
As a bit of an aside, I like all the innovation coming from Alpacka but I wish some of the ideas centered around reducing weight as opposed to making the rafts perform (and weigh) more like a kayak. Each innovation is really neat, but combined the boats are getting heavy. The new drysuit is a step in the right direction, as it could replace a spray deck and rain gear while enhancing shoulder season functionality. What I'd like to see is a 20-24oz paddle for class II river use, and a lighter packraft that uses lighter material on the inside of the tubes, maybe fewer seams, no stern attachments, lighter valves etc. With normal equipment (boat, spraydeck, paddle and gloves) you wind up at 8 lbs right now, which is a significant amount of weight.....it's more than all my other 3 season gear combined. A 4 lbs boat + 22oz paddle + 22oz drysuit and gloves would only be a ~6 lbs weight penalty if you leave your normal rain gear (~12oz) at home.