You want one that contains mostly Zinc Methionine and Magnesium Citrate as these are the most bio available forms of those particular minerals.
There are various forms of Zinc and Magnesium e.g. Zinc Methionine and Zinc Gluconate, Magnesium Oxide/Citrate/Stearate.
Your body extracts less elemental zinc (the stuff your body actually needs/uses) from Gluconate then Methionine. If I recall I think it's 30% for Zinc M and 18% for Zinc G...so you have to take 3 pills of something with 50mg of Zinc M to get 50mg of actual zinc....obviously more with something containing an inferior form of zinc.
Similar thing with the different forms of Magnesium, Citrate provides the most actual magnesium useable than any other form.
ZMA caps usually have B6 thrown in them because they are sold with the whole 'more energy, enhanced recovery' tagline (because you take them before sleep) but the B6 can actually end up giving you a counter productive buzz.
Cheaper in the long run to buy Zinc and Magnesium (preferably in Citrate) seperately.
Would also throw in being careful with Zinc supplementation, you only need about 50mg (of actual zinc, so if you're taking 200mg of Zinc oxide don't panic) max, Magnesium plenty of people are deficient in so once you start shitting as if you've had a night on Guinness then you'll know you're taking too much.
While we're on the topic of supplements - Swanson 5,000IU Vit D3, seeing as we getting into no sun season and D3 has a similar benefit to male vitality as zinc does, isn't going to hurt you or the wallet.