For competitive battle I say Amoongus wins for me due to 1, its poison type not only providing it utility in removing toxic spikes and immunity to poison and 2. Its abillity to be a mixed wall as well as a general supportive threat in many different scenarios. Although alomomola does also due its job vary well and survive in certain situations where ammoongus cant, I have had had more overall success with amoongus overall. However in the upper tiers(such as OU), I think believe neither of them can function at full potential due to prominent threats such as Tornadus-T, Thunderus-T, Rotom-W(to lazy to think of more atm) that can make their presence on your team, a liability and/or short lived. Also being generally outclassed in defensive roles doesnt help either. On the flip side, being able to halt certain physical attackers such as Landorus/Land T, Terrakion, Scizor, Mamoswine(alomomola), and Breloom(Amoongus) make them very valuable in those situations.
As for in game usage it could go either way really. I like that they are both very capable defensive pokemon and can prove extreemly handy in many scenarios in-game as well as being fun to use.
Lastly, Amoongus obviously has the advantage versus eachother due to being resistant to water, immune to toxic, and the obvious STAB super effective giga drain ftw gives it to Amoongus.
