Experience certainly makes for a smoother ride (heh), and it also means that Jack is even more aware of and excited by the subtle differences of being with a vampire. To be fair his kink for the new and different has always been a feature of his lovemaking. It's just that he got far less of it in his century and a half stuck on Earth - and none at all in the last six months in the Dome.
Which means the odd coolness of Spike's dick, where he's used to pulsing heat, is utterly fascinating to him and he can't help but want to explore that, getting his hand around it and squeezing experimentally, logging every twitch, every sensation, his attention to the kiss trailing off as he gets more and more intrigued by what he's got in his hand.
Until he lifts up with a slightly dreamy smirk, and wriggles back along Spike's body so he can get the more sensitive skin of his lips on that cool length, whilst also making good use of the position and tugging Spike's jeans down a bit more too.
((Ha ha - going commando will be yet another thing they have in common, then! *g*))