These two come up together constantly, and the honest answer is that neither is better. They sit at opposite ends of the texture scale and people have strong preferences without knowing why.
Mabroom: firm, slow, long finish
Long and slender with ridged reddish-brown skin, and noticeably drier at 2 out of 5 on our moisture scale. It chews rather than melts. The sweetness is a long toffee note that stays with you. Serve at room temperature: straight from the fridge it goes hard and loses most of what makes it good.
Kalmi: soft, balanced, everyday
A Safawi-family cultivar, smaller and cylindrical, at 4 out of 5 for moisture. Nothing about it is extreme, which is exactly the point. It is the one we reach for daily and the one we stuff with almond or pistachio, because it holds a whole nut without splitting.
How to choose
If you find most dates too sweet or too sticky, Mabroom. If you are buying for a household with mixed tastes, or for children, Kalmi. If you are gifting to someone who eats dates seriously, Mabroom is the more interesting one. If you are cooking, neither: use Zahidi.