I'm actually going with Volkov in this spot. I like Aspinall a lot and I think he's probably the best Heavyweight prospect out there, but he needed one more solid performance against a lower ranked gatekeeper before a push like this. I've yet to see him technically strike for three full rounds, much less five and in fact the momentum of the Arlovski fight started to get away from him slightly (whether due to adrenaline dump or an eye issue) before he transitioned to the grappling. Against a high-level technical kickboxer who's going to be willing to chop down his lead leg and rob him of his signature footwork and the power in his punches, that's troubling. Especially since Volkov has shown the ability to go for five hard rounds at an extremely high pace by HW standards and do it consistently.
And I'm not even convinced that Tom can go straight to the grappling. Volkov's defensive grappling is really not that bad. Curtis Blaydes set a Heavyweight record for takedowns (14 of 25) against him and the man still sits at 73% TDD rating. Marcin Tybura went 0 for 16 against Drago in their last fight and he's a credible chain-wrestler. Gane went 0 for 4. Overeem attempted one and didn't get it. Roy Nelson and Timothy Johnson both went 1 for 8.
But it's not just Volkov's ability to stuff the TDs; he's there to get taken down occasionally, as Curtis showed. Where Volkov really shines is making that initial takedown as difficult as possible, shutting down his opponent's attempts to advance position/rain down GnP/set up subs from top, and steadily getting back to his feet. This is a guy who got taken down by Blaydes 14 times and yet didn't get murdered with hellbows where most would have. He also had Fabricio Werdum on top of him for a significant amount of time and stayed safe long enough to return to his feet and knock him out. I don't see Aspinall repeating the performance he had against Arlovski.
I haven't seen enough of Tom to be convinced of his route to victory here against a proven commodity in Volkov who is capable of robbing him of at least some of his primary weapons.