Side slicers are a pair of blades that when mounted together represent an upside down "Y". There are variants of the side slicers including scoops and airplane wings. The other option is hammer blades. These are large forged or cast chunks of steel.
Side slicers seem to use less HP than the hammer blades. I have an 88" Alamo on the back of my 57hp machine (see note below) and it doesn't seem to notice the difference between tall and short grass. I usually go slow in the tall grass just to make sure I get a decent cut. The other factor that has a role in HP consumption is whether it's forward or reverse rotating. Forward (same as tire rotation) seems to use less HP as it is spitting out the cuttings as they're being cut. Reverse seems to use more HP as the blades will mulch the grass before it gets left behind, cutting it multiple times.
On the note of HP ratings, my MF 2706e is rated 57HP at the engine, 46HP at the PTO. I had to take it in for warranty work and they did a test on the dynamometer and it is actually 52HP at the PTO. This is a hydrostatic model. I'm guessing the PTO HP rating is with a load on the driveline, not stationary.
If you're going with a larger mower you may have to go slow if you bog down. I would recommend an Alamo because parts are very readily available. SHD88 seems to be the most popular model for the highway crews, at least in this area. Be sure to get a course cut model if you are just mowing to knock it down occasionally. The Ford 917 is also a good unit, but parts are not as readily available (had to replace my roller, but couldn't get one anywhere...). The JD I do not have experience with, but from the flailmaster catalog it seems to be along the lines of the Ford units. Flailmaster.com has everything for an Alamo that you could ever need, minus the main housing. If they offered the housing you could build a mower entirely out of parts from them.
Keep in mind when looking at used units that you may need to replace all the blades and mounting hardware and depending on the mower that can come in at about $300 or so. Also, check the bearings as those are not cheap. Alamo units are very expensive when new, but used you should be able to get one in rough but useable shape for about a grand (needing blades and possibly bearings). In good condition expect to pay upwards of $2500. They are definitely worth it as they are very heavy built units (the SHD88 is about 1,000lbs) and you can get all the parts you need to keep them running for the rest of your life.
