I'll echo what everyone else has said about going with
your heart, but here's a few observations I've made
having 2 Cairns and 1 Collie:

Big dogs are easier on the back where training is concerned.
Oz's nose is at hand level, so it makes it really easy to lure him
into position with a treat.

Big dogs are intimidating to most people, even if they
wouldn't hurt a flea. I'm not near as antsy when walking
alone with Oz at night, as I am with Murph.

Less worry about stepping on or tripping over a big dog
and seriously hurting him.

It's so easy to resolve any issue, by just picking up a lil' dog.
Oz got jumped by 2 big dogs recently. This situation
would have never occurred with Murph or Maddie. When I
saw the dogs racing towards us, I would have just picked
them up off the ground. Problem solved. I may just do that
with Oz next time too and just hope I don't throw my back out.

No worries about what you leave on counter tops with lil'
dogs.

Lower cost and less space needed for a lil' dog. I really wish
I had a lil' bit bigger back yard now that I have Oz.

Easier to get a small dog in a hotel.

All in all I think the pluses and minuses of each, evens out
and I've found I love both sizes equally.

Par...