It completely depends on the game location and condition. I pick and choose. I LOVE invisiglass, but not every game needs it. I have games that don't get any stray light on them, so a normal piece of glass works fine there. But other games need it. Plus some games like Tron just deserve it. I'm not going to deck out my whole collection with it, but where it is needed, it is amazing.
So are ColorDMDs. They are cool on every game I've seen them in. A very worthy upgrade.
This sounds like it is just a budget issue, so you will need to pick and choose for now. But I will warn you, it is hard to back out either of them. Once you go color, you will never be satisfied with monochrome. Same with invisiglass, all you will see is glare if you pull it.