My only experience is using Zendesk, and while they're great for large teams, their pricing tiers can be frustratingly orthogonal to startup growth rates. You might want to use them for the long haul, starting immediately if your budget can handle it, or transitioning later. If you're using SalesForce for your sales teams, there are support modules there that can handle basic ticketing. It all depends on what you expect your support demands to be in the future.