I have owned a RPM Max since they came out. My favorite river running front surfing boat. Fast for it's length, easy to roll and carves into turns. The only issue is the flat tail, which sometimes catches water and causes unintended tail squirts.
Never paddled an Axiom, have friends who own them and they love them for the same reasons they liked the original RPM's. Most of the feedback I get from Axiom owners over the RPM designs is updated outfitting and flat spins are possible/easier.
I own a Remix 79. For me all the river running benefits of the RPM Max with updated outfitting, plenty of room in the stern for gear you might need on the Grand and less unintended tail grabs. My RPM Max surfs easier for me than the Remix 79 but that is just my opinion. I tried the short playboats that allow all the tricks, but went back to the basic old school moves.
I have and recommend the Remix series what ever size works for you for the GC. Easy to paddle, comfy and fast. Even tho you will have rafts for support, it is good to have some extra gear with you just in case. Especially nice to have a break down paddle. Kayakers seem to explore a lot away from the rafts and I have seen instances where kayakers and their spare paddle in the raft were some distance apart when the main paddle got lost. A second water bottle is good to have as well. I kept regular water in one and flavored water in the other. You can refill both at lunch. A energy bar in your PFD pocket is a good idea as well.
Many paddlers do not realize that the Grand Canyon is mostly flat water. Mostly moving flat water but normally one or a bit more miles between the drops. There is also a lot of squirrely whirlpools currents etc to deal with. Plenty of rapids for sure, the most common rule of thumb for the Canyon I have heard is one rapid per mile. The Remix design handles all of the above well. I find the Remix is a great all around design for river running.
For the kind of river running I enjoy, all of the kayaks you mentioned would make me happy on the Canyon. Remix being a bit more comfortable and less grabby than the RPM Max and Axion in my opinion.
They will all be awesome down there. Having paddled them all I'd say if you are planning on paddling all day everyday the remix will be better for comfort and speed. If you are hopping in and out of the raft (which is what I would do) the two dagger boats would probably be better for the awesome big green waves to surf. If you are nervous about the whitewater the remix surfs great for how well it runs big water.
I was just in this situation last fall and opted to take a Mamba 7.6 (the small one). I'm in a weight range that makes that size slightly too small for creeking, but it was soooo much fun in the big water. Personally, I recommend folks take something with more edge to work with the big water and whirlpools. It was very easy to catch and carve many surf waves and the stern is stable enough that I wasn't getting squirted and pushed around like the RPM. Granted the RPM on the trip had super solid runs too, but I did notice that it came out the bottom of some rapids on the stern. I also think that the boat is very comfortable and I had no problem sitting in it all day. The current Dagger outfitting is really pretty nice and I liked being able to adjust the thigh-riser on the go.
I find it interesting how both here and on boatertalk the Axiom gets mentioned but the Pyranha Z-One doesn't. Very similar boats, the Large Z-One actually being in between the larger and the medium Axiom, sizewise.
Maybe not relevant, but I have no idea what you can get your hands on. So if you are considering an Axiom, at leats be aware of the existence of Z-Ones
I want to remind everyone that comfort is (probably) hugely important. Even if you're not in the kayak all day, on and off the raft, you're still likely spending many miles at a time in the thing. Stating the obvious, but many river runners out there are actually fairly similar so a lot of time the decision comes down to personal fit and comfort.
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