In 1995 by chance I came across a brand REEF from Brazil that makes both beach flip-flop and sandal. They have the kind of convertible sandal I was looking for. Quality wise I think it is as good as both the top brands, since my first purchase till now, I have owned 2 similiar pairs of sandal and the current has been with me for 10years. I only use this sandal during outdoor, beach or traveling so the wear and tear is rather slow.
So far both pairs I have owned did not breakdown on me under extreme usage, the only thing I realised was the sole worn down rather fast, maybe is my style of walking. They are very comfortable to put on and easy to covert to a flip-flop. The sandal is mostly flat but after breaking in, it will contour to your feet shape. Another thing I like is the weight, my current seasoned pair is 311g. It also packs very flat and it does not take up space in your backpack.
After 10 years of usage the sole is 50% worn down.
I have just checked the REEF website and found out they do still carry this sandal but slight different in design. There is a REEF dealer in Taiwan, not sure if they have this convertible sandal. At the moment it is retailing USD38 which is cheaper compare to the top brands. Previously I only paid US20 :-(
Tip: when crossing a flowing river, never use a flip-flop, my partner had her's swept away.