"Sounds like a 5in to 6in full-susser could be just what you need."
"Go buy an Orange 5 and get stuck in. Sorted"
I'm not so sure. You're used to riding a 4"? travel hardtail with fire XC 2.1"s on.
Do demo some full-sussers to give you an idea about what you'd be getting, but diving straight into the mid weight ones isn't going to give you the best impression.
100mm full sussers seem to have been marketted at XC racing in very recent years, though maybe they'd be more suitable if you don't want to suddenly find climbs being a lot more of a drag, and they'll still be very capable bikes downhill, just less so than higher travel bikes
(e.g. Santa Cruz Superlight, Trek top fuel, specialized epic, giant anthem X, marin (can't remember its name), scott spark, cannondale scalpel/rush, Kona Four)
Trying something with 120-130mm travel that has a sigificant increase in travel without losing too much climbing ability or gaining masses more weight would be sensible I reckon.
[e.g. spesh stumpjumper FSR, trek fuel EX, Gary Fisher Hifi, marin mount vision, giant trance X, cannondale rize, Kona One20]
Diving straight into 140-150mm bikes from a 100mm hardtail could well be a bit of a let down. Typically a lot more weight (than your merlin), shorter top tubes and slacker angles will compromise climbing and while they will descend incredibly well, you have to really want that to be able to live with the extra burden on the ups I think. Though they will still climb well, they won't by comparison to your merlin.
[e.g. Spesh Pitch/Enduro, Trek Remedy, Gary Fisher Roscoe, Marin RAGE, Giant Reign, Cannondale Prophet, Scott Genious, Kona Dawg, Orange 5, Commencal meta 5.5, Lapierre Zesty, Santa Cruz Blur LT/Heckler, Yeti 575 ...]
Having said all of that, a nicer fork and some tyres with a bit more volume may be all the change you need? Swalbe Nobby Nic 2.25", Panaracer Cinder 2.25", Maxxis Advantage 2.1" eXception 62a folding, Kenda Nevegal DTC 2.1", ... some suggestions