I have only used Avent bottles for my son (he is 5 months old and is our first child). At first I would forget to put the blue and white ring on the bottle and it would leak. However once I remembered to insert it (and make sure the top was twisted on properly, which isn't hard to do), I didn't have any issues with leaks. Any time we go anywhere I will have liquid in the bottles and I don't ever worry if it will leak.
Avent does a really great job of letting you really customize what you want to do as well as what works best for your child. I love the fact that I can interchange with other Avent products, such as the 4+ month sippy cup. The first time we tried the sippy cup it didn't go the greatest but he was undergoing day 1 of the final stages of teething too and I didn't realize that at the time. Then when I gave him his regular bottle, I just added the handles to get him use to the idea. The handles were a nice option but I prefer not to use them (so he's not picky when he switches to a cup). Two days later we tried the sippy cup again (but without the handles) and he took right to it (probably because I gave him medicine for his teething so he wasn't in pain)! However, I had to refill the cup/bottle three times because he drinks 9oz. Ha ha ha but it was his first time and we thought we would try 2oz first incase it didn't work out too well. Which brings me to my point of liking that they interchange, because now I can use a regular 9oz cup/bottle (which I already own) with the sippy top. I don't regret spending the money on the 4+ month sippy cup and only really using the top part because it was a great learning experience and we only bought the one to try it out first. Also if we have another child, then we will have an additional 4oz bottle instead of just one.