Full size RSK. I love the looks of the Deca but the Hogue RSK in my opinion is everything in a practical EDC utility folder the BM griptilian should have been. I love the orange colored one. I am sure the build quality, like on the RSK, is through the roof on the Deca as well since it uses the same lock and materials.
As far as the Mini (smaller version of RSK) RSK and the Deca, it would be a hard choice. You can't go wrong with either if the Deca is of the same build quality. The Deca is a little longer than the Mini RSK weighs about the same with the same thickness blade (0.094") but 1/4" longer and the handle is 1/4 " longer as well. I am not sure if the Deca really is that much bigger than the RSK mini. The non-clip point Deca (Wharncliffe blade) does have a compound grind with the bottom half being thicker and the top half thinner.
According to Nick Shabazz the bottom grind on the wharncliffe is reasonably thin and the top thinner making for a practical cutting knife unlike versions of this grind from other companies where both edges are unreasonably thick pry bar grinds making them less practical. Dual grinds do require dual sharpening angles. The Deca does come in a clip point version without the dual thickness grind but a standard grind. The standard simple grinds make sharpening more straightforward.
The RSK mini has a 2.9" blade that comes just under 3" while the Deca has a 3.24 inch blade. If you live in or plan to travel with your folder to places where knives over or at 3" are illegal then the RSK mini is a better choice. The blade stock thickness are the same on both.
Both versions of the Deca look super cool. The RSK has a classic working knife look. I can appreciate both.
Finally the Deca does have a tactical look (none of these three knives are designed primarily as weapons) to it that some of our more knife uneducated peers may find apprehensive while the RSK mini has a more basic utility folder look.