Ok let's assume Mundo has 1000 HP. His passive naturally regenerates 3 hp/sec at this amount of hp.
Add a Spirit Visage to this. 1250 HP means a base regen of 3.75 hp/sec. Now add a 15% bonus to that 3.75 and you're regenerating 4.31 hp/sec. Spirit Visage got you a whole 1.31 hp/sec. Your ultimate costs 250 HP (at max health) and regenerates 575. Cost: $1550
Buy a Giant's Belt and bring your HP up to 1430. Your passive now regenerates 4.29 hp/sec. Your ultimate costs 286 (at max health) and regenerates 572. Cost: $1110.
So basing the decision off synergy with Mundo's passive and ultimate isn't properly justified. I understand you get CDR and MR, but for a first item, I would much rather have some HP padding to allow rapid spamming of abilities on occassion or the ability to take a little extra damage. For the cost of a Spirit Visage, you could get a Giant's Belt and the SV's MR component for about the same price as a finished SV. Then the only thing left is CDR, which is not a big deal at this stage of the game. I would much rather build as quickly as possible into the item that feeds most effectively into Mundo's abilities.
A Warmog's brings you up to 1920 HP, which with the item's regen plus your passive amounts to 11.76 hp/sec. Your ultimate activates for 384 HP (at max health) and regens 768.... and these numbers will rise up to another 50% on top of this as you play. And the only thing you're missing is 10% CDR, which doesn't amount to much for anything but your ultimate, where it will shave off 7.5 seconds. To top it off, all this HP prepares you for taking the most advantage of another wonderful Mundo item: Force of Nature.