I don't think anyone can speak for what Doug feels, except Doug.
However, you might be better off asking your questions over on the KelTec Owners Group forums. Here is the link to the SU-16 specific forum:
http://www.ktog.org/cgi-bin/yabb2/YaBB.pl?board=105I would go for a C or a CA myself. The C can be fired with the stock folded. The CA cannot. The CA can hold a spare mag or two in it's stock. The C cannot. The only difference between the C and the CA is the stock.
An A would be OK too. That is the original. The A's sights were improved for the C and CA models. Also, the C/CA models have a closed ejection port and a shell deflecting bolt handle. So yes, the C/CA can be considered "improved" models, but it's not such a great improvement that the A is undesireable. The A is cheaper than the C/CA.
The B is the lightweight model. It has a thin barrel. You don't seem many people chosing the B option (at least I haven't)
The D's barrel is shorter than 16", therfore you need special government approval and pay for a $200 tax stamp to own it. The D is cute, but it's considered an SBR (Short Barreled Rifle) and you don't see many average Joe's willing to put up with the extra hassle and cost of owning something like that.
All the SU-16's I have fired have been excellent. They are extemely lightweight. So lightweight that the first time you pick one up you think it's a cheap toy. It certainly is not.