the claim for "flex boards" is that they have softer landings, loadable pop, pressable.
not everyone agrees that the landings and pop are better, but just about everyone will agree that you can really press a flex board (on rails or water)
I ride slingshot boards and personally agree with many of the benefits of "flex boards." Most importantly, i just have more fun on these styles of boards. they seem more forgiving to me too. the flex along with a featureless bottom allows me to flex and slide out of landings that are not perfect. Flex style boards are not for everyone though. like any board characteristic, they help emphasize certain styles. its something that you simply have to try to see if it is for you ar not. flex features determine how a board rides about as much as rocker type.
whatever board you get your gona want a 140cm or bigger i think.
Also you should look into getting new boots too. a nice new board aint gona be worth much if you are not comfortably attached and can controll it via that attachment.
If you're not ready for a flex board (SS recoil = best board for me), go for a LF Shane. I am just in love with this board! You can't go wrong with a Shane! Pop, soft landing, loose, good edge and speed, light, etc.
I bought a slingshot last year and found it wasn't for me at all. It wasn't that i didn't enjoy the board i just never figured out how to get good pop out of it. I get alot better pop on a foam core board. Its all depends on how you ride. It wasn't harder to ride but it was harder for me to get pop with it. I'm on a Ronix Viva now and im extremely happy with it.
I've honestly never ridden a flex board, but I'm not really one to press out lipslides etc. behind a boat. I'm more interested in good and consistent pop. I have been riding the b-side so far this season and just got to try out the Murray last Sunday at a demo. They both ride awesome but totally different. The b-side is a little grippier and edges slower but pops huge. The murray is quicker, looser and also pops big.
Hope that helps!