Canon and Nikon are the top two camera companies and it's doubtful that you would go wrong with either. They both have quite an extensive lens selection, so you will probably find whatever lens you're looking for with either brand. The 3rd party manufacturers also make lenses for both cameras, so you'll have those to pick from too. The 3rd parties don't always make each model available for the other brands, so that's something to consider when you're selecting a brand.
As for saving money by sticking with Nikon, I'm not sure that's true. I shoot Canon, so can't be sure about Nikon, but the Canon lenses for film cameras will not auto-focus with digital cameras. I imagine if your Tamron lenses wouldn't fit directly on a digital Nikon, there is probably an adapter available to make it fit - not necessary work like it did on the film camera, but would probably work, although AF probably wouldn't.
Jill, it's a big decision, but I'd stick with one of the top 2. I think Canon was top dog for a number of years, but Nikon has made some giant strides lately. I personally, wouldn't consider the other brands. Good luck with your choice Jill, and I'm looking forward to seeing which one you choose and of course, pics taken with the new camera.