I have spent lots at each and I think they are both good. I consider them equal. Where I live I don't have to pay tax at either. I just go by who has what in stock. Both will match each other's prices right there in the online checkout just copy and paste the address of the other vendor with the cheaper prices on a specific item. I just go by stock.
Also, I don't know what kind of transmission you are rebuilding or what kind of kit you are looking for, but in most cases I would not buy a TCI kit. Both Summit and Jegs both have house brand transmission rebuild kits for the common transmissions with Kolene steels and Red Eagle frictions and all the other good stuff just as good or better than any "brand name" rebuild kit for much less money. Something to think about. If you are looking at a TH400 or something common, chances are the TCI kit and the Summit kit are identical except for the sticker on the box and $100.
Also bulkpart.com is a very good resource with good prices. I have not ever bought a kit from them but I have bought late sprags and park prawls and things, other less common parts. They have a large selection and good prices but they are very very slow to ship. If you place an order, they might just ship it within a week.
