first off short board and long boards in the surfing world are nothing alike
^^ This is a ridiculous, clueless statement. For longer than I've been alive, shapers have been tweaking rocker/rails/fin setups/thickness/widths/etc & creating short/long hybrid boards that allow people to catch & rip smaller, mushier waves on shorter boards. Lots of people surf "high performance egg" & "Fish" shapes for that reason.
As far as difficulty, it's gonna depend on the wave you wanna surf, but overall, river surfing's a little easier, at least in terms of the actual "surfing" part of it. Once you figure out your spot, your entry, your downstream escape, & can stand, the rest of your ride isn't going to be as dynamic or reactive as it would be on an ever-changing ocean wave. Catching the wave is basically the same thing minus the whole "wave selection" part of surfing in the ocean -- paddle in, establish plane, pop, maintain -- but a main difference is that on the river you don't have a critical time constraint for establishing plane & standing. If you want to belly ride for 30 seconds before standing, you can do that, which you can't on ocean waves. At Lunch Counter in JH, you dive right onto the wave from the river left shore, plane out, surf.
Just like with ocean waves, you choose your board based on the wave you wanna surf. There's river waves you can surf with a traditional longboard/egg -- think wide, deep river & not a steep, pitching wave (Big Sur comes to mind). You could also take a thruster to a bigger, steeper, faster wave & do bottom turns & cutbacks.
Here's a story w/ pics & vid of a dude I know who likes to surf Skooks: a few words with safety man Steve Fagan on Bathtub Racing, the Billabong Odyssey and surfing Skookumchuck with Elijah Mack - Surfing Vancouver Island.com