I just did the opposite switch, so i can compare the 2 pretty easy. I had a 2005 Avalanche Z71 with only 59,000 miles on it. I loved the truck. My main reason for the switch is because I have 2 small kids...more about that in a minute.
First off, the Avalanche, is more than capable as a truck. Many "Truck" guys look down on Avy owners like you don't have a real truck. The cab is just like any other crew cab truck. The bed allows you to do a lot of different things. Ivd had many different chevy trucks, and out of all of them, the AVY was the best. Sure the bed is a little short, but how often do people really need an 8 ft bed. For those few times you need it, just fold the mid gate down and you can fit longer stuff in. The covered bed with locking tailgate gives you so much room for cargo. With the bed covers off, the truck is a beast and can haul anything. I once put 40 bags of mulch in the bed and it barely squatted. It never got stuck anywhere and made an awesome truck.
The Avy could easily seat 5 people and had tons of cargo room that was dry and lockable. The reason I had to get something bigger is because of the configuration of the back seat. I have an 8 month old and a 2 and a half year old. Both in car seats of course. My oldest daughter sat behind the passenger seat, the baby was in a carry type car seat with the base mounted in the middle. The AVY does not have the car seat hooks behind the driver seat. So to use 2 car seats, they have to go side by side. It works fine for now, but when the baby moves to a bigger seat in a couple months, they won't fit next to each other. So i decided tp upgrade to the Tahoe. One the Avy, even with the 2 car seats side by side, i could still get an adult passenger in the back behind the driver. So I could take 3 adults, 2 kids and TONS of cargo on a long drive vacation.
Now the Tahoe.....I opted for the 2nd row bucket seats. SO, now I have room for 2 front adults, both kids, and realistically, 1 adult in the 3rd row. That then leaves me almost zero cargo area. With the 3rd row being used, i can't even fit a stroller in. In all honesty, i had a ton more room in the AVY, and could still put 5 people in it. The Tahoe with 5 people has no room for anything else.
I really like our new tahoe and it is great for the family, but i really miss the Avy. Go for it, you won't regret it.