I have used both - I had TT closer to nature with DD because I had a TT breastpump (totally useless breastpump i have to say) but the bottles were fantastic and never had a problem with them.

I used avent with DS because I bought an avent breastpump (brilliant breastpump) but I have been not so impressed with the bottles because they do leak around the seals of the teat.

Both my kids have ended up on formula before 6 months old so the bottles I have do get a good workout (after using bottles for DD, I bought all new ones for DS because they go a bit yucky over time).

There are less teats to buy for each stage with the TT ones (stage 1 - up to 3 months, stage 2 - up to 6 months and stage 3 - beyond 6 months). Avent does 4 teats (stage 1 - up to 1 month, stage 2 - up to 3 months, stage 3 - up to 6 months and stage 4 - beyond 6 months).

TT bottles are a bit of an obscure shape so can be a bit difficult to fit into bottle carriers - avent are more straight up and down and not as bulbus.

Apart from those minor differences, I thought they were both great bottles but TT wins everytime because of the no leak thing (Avent emptied a whole bottle out through my nappy bag before I realised they leaked and DS used to end up with it leaked down his clothes if I hadn't realised it was leaking - I'm glad someone else had trouble with them leaking, it's not just me being unco).

Hope all that helps.