Until you go pro, its possible the Raytech Tumble Vibe 5 is the best deal. Its got good capacity, can handle some pretty good sized rocks (if properly mixed with smaller stuff and loaded correctly). I have one that I've used a great deal for more than 4 years.
I also have the small Gyroc (Model B). Its also good, slightly smaller, tends to slip its belt or be more finicky about how many rocks it will jiggle. Its not as easy to clean out since at least on mine the bowl is difficult to remove.
A real work horse also small in size but lacking the center cone compensates for that size is the mini sonic (by Diamond Pacific). It has a slightly different action (rolling in one direction instead of the vortex rolling of the others), but it really moves things and works well. Mine is the older model without adjustable speed control. Any one of these three machine is a reasonable starter but if the price on larger machine is not a putoff, I'm sure they will do as well. I have yet to hear anyone say one of the vibes from Harbor Freight (that are not recommended for rocks) is performing well for them so that one is not on my list.
I may have garbled the name on the mini sonic machine and will correct this post if confusingly so.
I might add, I process lots of odd shapes, often preform my stones on the grinder/cab machine before running them, and often run beach or river processed stones rather than broken stuff that would normally require rotary prep and all of those facts may make it easier for me to function with these entry level machines. BTW two were bought used and each has its own advantages.