I agree with Kevin. The ST-35 is $125 to $150 less expensive but has only half the power of the ST-70. The ST-35 has a few other disadvantages.
1. The ST-35 has no power switch so unless you want to add a power switch or plug the amp into a switched outlet you will wind up having to plug in and unplug the amp every time you want it on or off.
2. The ST-35 can only use EL84 output tubes while a modern ST-70 kit can use EL34, 6L6, KT66, KT77, KT88 or 6550 output tubes. Each tube has a slightly different sonic signature and you can tailor the sound to your liking by swapping out different types of output tubes.
3. In stock form the ST-35 has no adjustable bias system. You really need a matched quad of EL84's in an ST-35. As the tubes wear there is no way to compensate for tube wear by readjusting the bias like on an ST-70. That said, Dynakitparts does have and adjustable bias system for the ST-35 that you can add to any ST-35 to alleviate this issue. See link below.
Dynakitparts ST-35 bias control kit