It really depends a lot on what you are actually looking for:
If you want a fast mover, the A-10 is not for you. If you want A/G, the Mirage comes last. For A/A on the other hand it's the obvious choice.
Systems wise the A-10 is rather complex, but it's all very logical. So it's not that it's very complicated stuff, it's just a lot of it. ;) So it's rather a quantity than a quality issue, if you will. Just make sure you have a HOTAS with tons of buttons. ^^
Viggen and Mirage systems complexity is pretty similar IMO, although they feel completely different. Mirage is more digital while Viggen is analog.
When it comes to simply flying Mirage is easiest thanks to FBW, A-10 is also a piece of cake and Viggen takes a bit more practice. But since you already have experience in FC3 that shouldn't be a big concern.
Hope this helps and welcome to the forum!