All down to which dog you like best id say. You will get good workers in all the types, just because you see photos on here of beaten up Patts, dont mean to say they are all like it. One of the hardest dogs i seen was a russell bitch.
Just got to be careful when your looking for pups though, especially russells. they come in alsorts of shapes and sizes and there is alot of shit out there.
Fells in my opinion is a rough coated black type. Drives me mad when my bitch gets called a patterdale because they are generally accpeted by most as the smooth caoted black dogs