I've tried various casters, spiked platorms, etc... and found no audible difference between locking casters and spikes. The casters act almost like spikes, putting the weight on a small contact area. I did make sure when buying the casters, though, that they locked tightly; that is, they didn't "wobble" after locking. This way they will "anchor" better, allowing little or no vibration to be used to move the speakers. It also keeps the speakers from possibly drifting across the floor and out of their ideal position over time, due to vibration.
Casters/wheels are made of different materials, such as metal, nylon or rubber. Theoretically, this can make a difference, as each of these each have different vibration-transmission characteristics. Metal wheels will anchor the speakers better to the floor (unless they slide easily against it). Rubber wheels will absorb some resonances through "jiggling" themselves, but theoretically that can lead to slight smearing of the sound. Nylon is in between metal and rubber in its hardness/resilience, so it has a bit of both characteristics. You may wish to try experimenting to see which type works best for you.
Honestly, I found a lot less sonic difference when I did my experimentation than I expected. I'm sure these things affect turntable performance, but perhaps the scale of vibration is different enough with large, heavy speakers that it swamps the minor difference wheel material makes.
The change in height of the speakers seemed to make a bigger difference than the type of wheels. Height above the floor can have a big impact on bass, especially. As long as the speakers were at the same height and the wheels locked, there wasn't much difference between wheel materials, but theoretically there should be, so it doesn't hurt to experiment. YMMV.
Whatever kind you use, screw the casters securely into a nice base platform, then put the speakers on that. You can experiment with gaskets and other absorptive dampeners between the caster base plates and your platform, and also to couple the speakers to the platform. Blu-tac is often recommended for the latter purpose, and not without reason, but (again depending on your circumstances) spikes may work better.
For me, with Infinity RS 2 speakers on a linoleum-over-concrete slab floor, the easiest/best solution was to use locking casters, a heavy platform (plywood glued under marble slab), and blu-tack between platform and speakers. I further anchor the speakers with a paving block of concrete placed on top of the box, both to add mass and further "anchor" the speakers. In my case, the block is wrapped nicely in a towel covered with brocade cloth, both for appearance and to protect the speakers from scratches --and also because there are rear-facing drivers in this model, and I want to limit reflections off of the (now closer, thanks to the paving block) box top below them.
If your floor is smooth and hard like mine, you may also want to experiment with putting a thick towel or throw rug down in front of the speakers, to absorb some bass reflections. In my situation it helped when the speakers were on the floor, but is less significant with them raised on the platform/wheels.
I use a builder's laser-line leveler to position the speakers quickly and easily in the exact same spot each time. Barring that, you can simply put some kind of mark on the floor. Tape works well if you don't have carpet, and is easily removable.
There are always trade-offs and compromises with these kind of things, and sometimes surprises in what works best, as external things like room resonances and reflections may come into play with speakers, so EXPERIMENT to find what works best in your case. Ultimately what matters most is that you enjoy the sound you are getting, and only your own ears can tell you that. Have fun and good luck!
