Used TechDAS Air Force III vs New Feickert Firebird with Kuzma Safir 9 vs SME 20 MK 2

@clearthinker I am not any kind of engineer nor do I have enough physics knowledge to try to answer your question. However, I do know what I hear (and am fairly confident in not falling victim to owner's bias) and I consistently hear what I described earlier.

For a truly technical point of view, Fremer's review is quite complete: https://www.theabsolutesound.com/articles/kuzma-safir-9-tonearm/

I like the idea of the Kuzma Safir arm

+1 it's an end-game arm, nothing to upgrade to

Understand the arm is next level but would the Stabi R be holding it back too much....?

As a suggestion, in relation to having two TA's.

If the Safir Model is selected, this will have a Fixed Headshell and leave the situation where wanting to trial out Cart's or have a collection of Cart's at hand to use, is presenting a precarious and risky situation each time a Cart' is to be swapped out.

A second TA worthwhile considering is one that has a Removable Headshell, but even more importantly, a design that has a very rigid secured method put in place for the Head shells attachment. A interference Fit design is the way to go.

As a secondary consideration, a Headshell Design that fits the above and is also available in a variety of materials.

With this type of TA in use, the Headshell can be pre mounted with a Cart' which is then aligned. Once in this condition, it can be kept in a Cart' Keeper. This is how I am doing it for myself. 

The specific changes required between Cart's for the TA to undergo during Cart' Alignment can be recorded.  This as a method will enable Cart's to be pre-mounted onto a particular Headshell type and put into service as a very low risk operation and easy to achieve method.

It will certainly leave the main TA un-tampered with, keeping in place the critical adjustments that have been carefully added during the periods spent making discoveries, in conjunction with the Cart' of Choice.

As you are expressing an interest in having Two Phon's as well, the method will also enable speedy setting up of exchanges, to create the initial impact of the changes being made to not incur too much delay during the comparisons. 

OK. Items I want on a turntable are an isolating suspension, vacuum clamping, a magnetic thrust bearing and a versatile arm I can adjust to the majority of cartridges. In terms of clamping, reflex clamping is a second best and a good standard thrust bearing is tolerable. The other traits can not be compromised. 

The Air Force Three is a complicated used turntable with a second rate arm. I would not even look at it. The Firebird and Kuzma R do not have suspensions so you would have to spend an additional $5K on a MinusK platform. 

The Ref 2 and SME 20 are very similar tables. Both have a suspension and both utilize reflex clamping. The Safir regardless of anything anybody including Frank Kuzma says is viable for very low compliance cartridges only. It is otherwise well designed. If you are going to spend the rest of your like with a Koetsu, fine. Otherwise you need to look elsewhere. The Reed 2G, Tri Planar, 4 Point 9, SME V and Schroder CB are all excellent arms. I went with the Schroder as did Mark Dohmann. He demonstrates his Helix turntables with a CB. 

Of your selections I would get the SME and put a Schroder CB on it. My next choice would be a 4 Point 9. The Safir is Frank Kuzma doing his "just because I can" act. 

A Sota Cosmos offers Vacuum clamping, a great suspension, a magnetic thrust bearing and the CB fits perfectly. On top of this you get state of the art speed control and a great isolated hinged dust cover you can use during play.  Just an FYI