From the title, I thought this might be a discussion on correct nomenclature!

I like both. My EDC is a Colt DS, and the gun I take to the city or on the road is a 39-2.

So, in answer to your question, if I had to pick one or the other, it would be a 2.5" Model 66. Or my 39-2. Or my BHP. Now that I think about it, I've NEVER been able to choose just one in any of these "what would you choose" threads!