I am no professional, but we use the Andis 2 speed and have good luck with them, although I have only ever used the highest speed...
I clip a lot of dogs, so we have 2 Andis clippers (well 3 but I don't use the 3rd...can't recall the brand) I use one until it gets hot, then switch using the fresh (cool) one, then when I switch again, I usually change blades because they are still hot. I use the Cyrogen (sp?) blades, they are far superior and stay cooler longer.
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I would think that cordless would need to recharge before you are finished...it takes me quite a while to do a full clipping on one dog.
You might want a smaller one to do their feet, something that will fit between toes/pads.
Be careful...this is true...I know of TWO people who have cut off nipples when shaving their dogs!
Also, be sure to touch the blade often...they dog get really hot and especially if you are clipping their tender belly or sanitary area...you want the blade to be cool.