Bows and SLE's have full bridles, hybrids allow both or use a 5th line to support the leading edge but thats arguable (like the rebel which some still think is a C and others call a hybrid)
The reference to the new ultra flats is a complete design difference from the bow/sle kites rather than just a very flat bow/sle kite. The biggest noticeable difference is the extra strip on the struts you can see in this picture.

There are only limited ultra-flats as far as I know, I think wind-x (the moonsoonz referred to in this topic) and fluidkiteboarding is due to release one soon (an australian brand). Its a new concept from Bruno so not everyone has started using them yet. I've heard rumours that Cabrinha has one coming but no reliable sources have confirmed them yet so its still just rumours.
If you are looking at only second hand, then this probably takes the ultra flat option out of the equation because they have all just been released, and to be honest, there aren't enough reviews out yet to give the public an idea on whether they really work as well as the manufacturers say. You might be able to source a demo one, but you'd have to speak to both guys to see if thats possible.
Of the bow/sle's/hybrid's, I have heard good stuff about a couple of them, and you'd probably need to look at the 13-14 range to match your requirements (i.e. to match the 15 but keep the upper range of your 13m). Some kites like the rebel are great but lack the low down grunt of some of the others like a Switchblade2 so you may have to opt for a 14 rebel if you go that route. Rebel 14 will still outturn most of the smaller SLE's. Waroo 13 would work well, havoc 14m is good and LF have some great deals on demos at the moment, Switchblade 2 12m is very grunty and popular. Heard very mixed reviews about the link. Its a good year to buy second hand, not a lot of duds.