I've had both the Sota and SME 20 in house. The SME 15 would be a downgrade, the Sota is on a par with the SME 20. If you want SME you really want at least the 20/3 or a 30.
The Techdas is a rehashed Micro Seiki 8000 - which some still prefer to the Air Force 1. In my view the sweet spot for the Techdas is the 3P - significantly better than the 3 - do note there have been the odd problem with these decks - motor issues and speed logic issues so they are not immune to issues.
You will get lots of alternative suggestions, but I recommend having a look at the Kuzma R or M with the 4Point arm - much better sounding than the SME range from my personal experience of listening to both for a few years. The Kuzma's are fantastic and more importantly very very easy to set up accurately with little experience. I have been listening to a Kuzma M/4Point/van den hul grand cru for a while now - for the money its superb.
Finally your last option would be to get your Sota fully refurbished by Sota and brought up to new, you can also get your SME fully refurbished by SME along with upgraded wiring and V bearings. This would give you a superb front end by any standard if you are attached to it.