Hmmm. Like everything else you get what you pay for. The Weber kits are fairly inexpensive, but they don't have the best components (bad jacks, not so great caps, etc). The Mojo tone ones are fairly expensive, but trend toward cork sniffing. Between the two I'm liking TubeDepot for a good compromise, but you can build a great amp with the Weber kits if you know what's good and what to replace. Also, amp kits vary in price due to the size and power of the amps in question. For example, a tweed 5F1 is not going to be the same price as a Marshall JTM45.
What amp were you thinking of building? Maybe we can guide you better knowing that.