It depends on what you want it to do. The Combo 2 looks like a rework of the Limitless, of which machine I have 5 or 6 copies and like them a lot. It's a traditional side-air RDA with a really big juice store, is all. Real versatile; build it any way you want. Cloud, flavor, big air, tight draw, whatever pleases you. It has a V-style deck and will be easy to build.
The RDTA 5 is something different. It looks to be a try at a full blown cloud machine. It had independently adjustable bottom and side air, in the manner of competition toppers. I wouldn't expect huge flavor out of it, but if you close the air down some it ought to be at least OK. But if it's cloud you want, you should get more out of the 5 than the 2, at least if you know what you're doing as a builder. Whether or no it is actually good at it is a question I can't answer, but that appears to be what the designers were shooting for. Note to the inexperienced: bottom air toppers tend to leak. Make sure your coils are wider than the air intake port.
Now, I say all that, but I don't have either machine. I am going on the pictures on iJoy's website. YMMV, but I'd pick the 2 as it better suits my style of vaping.