Miss Tilney is probably my favorite minor character of any JA novel (and I say "minor" only because I couldn't think of a better way to say "not the heroine but an important character nonetheless"). For me, Isabella is an exhausting character. If I knew her in person, I imagine I would get tired around her. Miss Tilney on the other hand has a reserved-ness about her. She's not standoffish but she has no need to overly exude her emotions to get people to like her. She's likeable from the beginning. And you're right; she's an excellent offset to Isabella.

In Chapter 8 JA says of Miss Tilney:

Her manners showed good sense and good breeding; they were neither shy nor affectedly open; and she seemed capable of being young, attractive, and at a ball without wanting to fix the attention of every man near her, and without exaggerated feelings of ecstatic delight or inconceivable vexation on every little trifling occurrence.

If that isn't a direct comparison to Isabella I don't know what is!

Another thing I like is that the friendship between Catherine and Miss Tilney begins in Chapter 8, slowly.

"But the hindrance thrown in the way of a very speedy intimacy, by the frequent want of one or more of these requisites, prevented their doing more than going through the first rudiments of an acquaintance, by informing themselves...."

JA shows us there does not need to be an immediate and hasty flight of intimacy but that genuine relationships take their time. It's a theme we'll notice more as we continue on with the reading so I will stop here before I go too far :)