Hi
@boxing, I am in a somewhat similar situation as My Animas pump is being phased out.
In Australia I have the options of transferring to a Tandem SlimX2 which is being phased in, in place of the Animas, or switching to Medtronics 640 or the Roche Spirit.
The Medtronic 715 is older than the Tandem slim as far as i can tell although the Medtronic 715 is apparently compatible with a number of CGMs. see loopkit.github.io - Loop Docs. Is it a matter of supply to South Africa that a more update Medtronic pump is not available? I could not find any reviews that comparable the Medtronic 715 and Tandem Slim x2 directly.
The newer Medtonics have all the features which will not be available in Tandem Slim (in Australia anyway) until late 2019.
Tandem slimX2 has compatibility with Dexcom see askmanny.diabetesblogs.com Review

Tandem Tslim x2 with Dexcom 5 integration. (US only, I think) And the advantage of updatable software without need to change hardware. (? due for rollout next year to Australia and ? elsewhere) as well as a later roll out of dose reduction software feature for low blood sugars, integrated with CGM. (Which would bring it up to date with the current Medtronics 640.
see also: testguessandgo.com - A 5 month review of the Tandem:t-slim x 2.
Reviews on the Tandem Slim are mixed; see forum.tudiabetes.org - Tandem Pump, convince me why not -Jan 17. problems with the belt clip arrangement and some features like not being able to over ride the pump's recommendation about Insulin On Board are noted.
So older Medtronic technology to newer less proven Tandem? I would like to have a better comparison option I must admit.
At least the Tandem is more easily upgraded once purchased. And personally I like to back the underdog, as having Medtronic too dominant in the pump market is, to my mind, not a good thing. But what is best for you or me ? That is the real question.
I do still have a challenge ahead with weighing up Medtronic 640 against the Tandem Slim x2.
Best Wishes with whatever you decide.