I own the Ti15 since not too long, and I went out and tried the Ti18 in the store. The Ti18 is smaller, lighter, and stronger, but not notaceably much. Noise level also about the same.
If you have a Ti15 and you're happy with it, why get a new one? Totally unnecessary.
But the 15v platform is dead for Festool, so if you plan on future upgrades to your cordless tools 18v is the only way to go.