Hi and welcome to the board. Both of those should be good to excellent choices. The MVP 3.0 Pro has been out for a while and gets great reviews. The Siggie 75 is brand new but, like the Innokin part, it comes from a well respected company with a solid rep for quality. Let me give you a couple points to ponder:
1) I don't view those as being in the same class, really. I think of the MVP as being a large and powerful convenience device and the Sig to be a small, lower powered, but top grade mod. That's not to say the MVP is bad gear, but rather it had a different design philosophy. I feel sure the MVP will be easier to use, if you don't mind being tethered to the wall while it charges. Getting the Sig is not as "plug and play" as the MVP will be, you have to carry spare batteries, etc.
2) I am personally sold on replaceable batteries. I like IMR chemistry better than LiPos-- they're safer-- and I like the convenience of slipping in a fresh battery when the old one dies, and recharging when it's convenient for me, not the mod. I further like the use of a stand-alone charger. I think they take better care of your batteries than any dime-sized convenience board can. Batteries are expensive to replace and dangerous when mishandled. This just makes sense to me. I also like removeables because an on-board battery means it's a disposa-mod. When the battery goes south, you throw the whole thing away and get a new one. WIth replaceables, you just get another battery, and can continue using the same box and electronics. And back to the charger, if you buy a good one-- I use the Xtar VC4-- you can keep an eye on your batteries and monitor them over their useful working life for capacity and voltage, replacing them *before* they go out, so you don't get stuck the night before vacation not having any batteries to take.
3) I think TC is the coming thing. I wouldn't recommend anybody spend that kind of money on a mod nowadays without the temp circuits. I have two--a Snow Wolf and a SX Mini M-- and I love them all to pieces. I still have, and love, and use, all my old mechs. And I might one day pick up a box or two that is only VW to round out the collection and because I am queer for gear. But if you don't have a lot of mods laying around, go for the TC.
4) I don't have either of them, and I want both. You make the call as to which one first