Our latest rig came with #20 tanks. At first we were concerned but honestly, we typically get 6-8 weeks out of a tank (more in the summer when it's only for cooking). When empty, we typically switch to the 2nd tank and start keeping an eye out for a fill station. No real rush.
Honestly, prefer to have them filled rather than exchanged as you get more propane for less cost but it's not a huge issue. Maybe $20/yr overall cost difference.
One nice thing with the #20 tanks is if the fill station gives you a hassle over the tank age, just do an exchange, checking that the new tank has a few years left on the certification.