Most of the time they are used interchangeably but check with your doc.
I have noticed that Gonal-F had (maybe still has?) promotions where if you end up needing to do multiple cycles, they cut you a discount/rebate of some sort. So you could save money that way if you end up needing multiple cycles.
Also, both Follistim and Gonal-F are VERY expensive......So, if you *choose* follistim when your insurance will only pay for Gonal-F be prepared for a hefty bill. That's why I say first check with your doc and then verify *in writing* from your insurance if they say they will cover the follistim, too.
I have only used follistim (wish I would've used Gonal for the $ savings as I'm OOP) and I can say that it's super easy to use. The needles are very fine and it travels well.
Also with follistim, the ampules come with a signifigant overfill. For example, on a 900 ampule, the overfill is approx 150 (1050 total, approx). This can save you some big bucks if you have to pay for some/all of your meds and plan it out right.
I don't know about overfill, etc, with Gonal as I haven't used it.
Good luck with yor meds. I know it can be confusing.