I'm pretty sure that you should get the unvented type. The fact that this one specific line (out of three in total) going into the floor is not connected, does not lead to problems because, if I understand correctly, it only functions as a sort of "emergency-overflow-of-last-resort" in case the overflow tank is not large enough. I may be wrong on this though. You may want to get the line going through the floor, with the funnel-shaped end, once you are at it. Replacing the rubber hoses/clamps for the three lines is also a good idea if it has not been done in recent history.
