Does riot let you choose to give both a counter and haste, or only one?

Riot is a static ability that gives you a choice when the creature enters the battlefield: you may put a +1/+1 counter on it, or it gains haste until end of turn. You pick exactly one of these two options — the rules do not allow you to choose both. (CR 702.138a)

The choice is made as the creature enters the battlefield. If you want the creature to be able to attack immediately, choose haste. If you prefer a permanent power/toughness boost, choose the +1/+1 counter. The decision is yours, but it is mutually exclusive.

It's worth noting that if a creature somehow has riot multiple times (e.g., via a copy effect or multiple riot instances), each instance of riot is applied separately, and you make a fresh choice for each one. In that case you could end up with both a counter and haste by choosing differently for each instance. (CR 702.138b)

Example: You cast Frenzied Arynx, which has riot. As it enters the battlefield, you choose to give it haste so it can attack right away. It does not get a +1/+1 counter. Alternatively, you could choose the +1/+1 counter, but then it would not have haste and could not attack that turn.