A halfway decent advisor will tell you that if you want to exercise your options (at which point you have a taxable event) to sell enough stock at that point to cover the taxes. If you want to hang onto the remained because you think your company is poised for growth, great. We always sold options when we exercised them - which wasn’t always at vesting because if they are under water at vesting you are buying something for more than you can buy it on the market.
Grants are just given to you. Grants we always set aside the money for taxes when they vested as well - either by selling some of the grant or by setting the money aside. Grants we sometimes held onto for years when we felt the company would do well. (And sometimes this paid off very well, and sometimes it didn’t). Grants were ours though, so once we had them, we could leave the company and retain them.