The Tavern

I have an amulet of guardian spirit, but I found vitality on sale. I am playing a heavily armoured, melee DsCK. Should I buy it? (I currently have some 19 MP and 144 HP).

19 > 15

19>15, but the amount of damage taken from MP pool is based on the HP/MP proportion. Less damage goes through MP if the gap is large. Personally I'd go with Vitality.

I believe it is optimal to compare HP regeneration with vitality and HP/MP regeneration with spirit and use whichever is greater.
At least if you are not expected to die in just a few attacks, then guardian is obviously better because regen barely matters.

Guardian spirit is usually better for this kind of character. Your mp regen rate alone will wash out the advantage of regen from vitality and you have more than 15 max mp.

Buy it and check which one you like.
You might get the mutation 'mp-powered wands' and realize very lately that without mp you cannot use wand anymore. Did cost me a char once !

I would generally advise Vitality, because it is a more sound strategy to keep your mana on hand for at least the occasional emergency invocations or low level support spells. If you have absolutely nothing to spend mana on, then perhaps guardian spirit. It's not really a total deal breaker, but one other consideration is that you can have your mana fully drained, and vitality will be nicer in those situations.