I have not built a tank out of either but have done allot of research on both (as I am planning a 2000g tank) and have worked with both in many other applications.
Epoxy is great if used properly. I am guessing that people complaining of its brittleness, did not use reinforcement cloth for the entire tank. The cloth is not very expensive and if you use epoxy as you would fiberglass resin and embed cloth in it, it will be very strong and should last a very long time. I would not use epoxy just painted over the wood without cloth. That is not a very good way to do it.
I would say not all epoxy is created equal but I would not be comfortable suggesting a brand, though it seams SweetWater is used allot around here.
Liquid Rubber, I personally think is a good choice for several reasons. First is the ease of use. It has very little fumes, and you don't have to rush to get it applied and is just as easy if not easier to work with than epoxy. I would say it is cheaper when compared to a good epoxy job done correctly. You can use fabric to reinforce it making it very strong yet flexible. Because of its flexibility, moving a tank around should not be an issue at all. The only draw back is that it only comes in black.
I can recommend a brand here, it is called Wetsuit. It is distributed only by Neptune Coatings. It is a neoprene rubber that has better characteristics over most other liquid rubbers. I work for a roofing consultant firm and we have allot of experience in liquid rubber coatings for roof waterproofing applications and Wetsuit is hands down the best liquid rubber product out there. There is no question in my mind.
Wetsuit was designed for commercial roofing applications but has been used for several other things including ponds. They actually just dig a hole and spray this stuff directly over the dirt. I know of a pond done this way and it is almost 10 years old now, with no problems. It bonds very well to wood and silicone bonds well to it, I have personally tested this. This company also sells reinforcement fabric.
I do not know of any plywood tanks built with Wetsuit yet but I have allot of experience with this product and will likely end up using it for my build next year.
If you decided to go with it, you would want the 1 part Wetsuit and Invisilink is the reinforcement fabric which comes in a few different dimensions. I have allot of hands on experience with this product and deal with the distribution company because of my job, so if you have any questions, let me know. I don't think they have prices listed on there website but I can get pricing for you if anyone is interested.
I personally think this stuff will be the next big thing in making plywood tanks.