I LOVE Aussies -- just saying (since I have had three). We have one (a Champion on the way to granding and intended Westminster goal according to the breeder) in class that I drool all over every week. We have a red merle too that is nuts (but only when Mommy handles -- solid and behaved with Daddy -- hmmm). Just like anything, breeder makes the difference. I have watched a breeder in So Cal for years with dogs really solid physically and mentally -- but when it came to the wire I was taken by my rescue sheltie girl rather than geting one from this breeder I like. I wouldn't rule out an Aussie/sheltie mix if there are some who need homes, just thoroughly evaluate the litter and choose wisely.
As much as I needed the 2nd dog (sheltie) to further my goals of training, rally, etc., I did like the single dog dynamic better and I think our future will be a onsie next go-round.