hello, you should be fine with the towing of both boats, however may find the car working hard to get them up slips! just dont wear your clutch out
now we have the humber ocean pro 6.3m, and are very happy with her offshore capabilities, which handles the scot west coast, larger wave pattern very very well! in fact she prefers it to the stuff we get in south wales (short steep choppy stuff) and is a joy to drive in these conditions. we have a suzuki 140hp on the back which takes it to 40 knots, and balances the boat nicely, didnt go for higher hp as this would add significantly more weight to the stern. we tow it with a nissan x trail which does it nicely, would even mroe prefer a 4x4 with low box gears to help on loose ground, where the nissan struggles if you dont keep going and get stuck! hasnt happened
i think you will probably find the 7m better simply because generally the bigger the boat you get the better its capability, however im very happy with the 6.3 and think that the bigger you go, apart from on the water, it gets tricker on the road, and seeing as you are using 2 wheel drive, i would stick to the 6.3 which with trailer would come in at just over 1000kgs depending on what you have fitted.
have you ever been to achmelvic beach, near lochinver? thats where we go every year, amazing place to go cruising!