As you said its a huge price difference, the positive points swaying me to the biomaster are the integrated heater, the simplicity of the pre-filter and how easy it is to help maintain it well. The warranty of 3+1 years and the overall reviews and comments that people are raving about it. Its been in the market place for quite a while now, and the product is solid and reliable in general. It seems like there are more happy customers than not. The AquaEl on the other hand being Polish Im worrying about parts, warranty and how simple it is to remedy problems as and when they arrive. I think the fact that I can hide a heater that wont effect flow, and have something thats easier to maintain in the long run is a huge bonus. I just dont know if if worth paying double the price. Yet I read a review today on the AquaEl in PFK that gave it a glowing report, runs near silent (very important for the other half) top marks and highly recommended. Likewise, the pond guru guy ( though I hate his sales patter for Biohome blah blah, the less said the better) he had nothing but praise for the solid build quality and said that the filter is very good value for money and like the pre-filter idea.
I have had JBLs before. Infact I have a 701 that is brand new and never been used. They are as you said pretty good all-rounders, and the price is very attractive. I especially like the fact that it onlys draws 20 watts power, and again are well known for running super silent, where the 850T is 32w. Thats a big difference in power consumption per year.
I spoke with @wookie over PM, and he gave me some sound advice and honest info on his 2 Biomasters he is running. I thought after that chat I had made my decision, then a few more comments on here and I doubt myself again. You guys dont make it easy for a fella lol. I could quite easily buy something like an AquaEl Ultramax 1500 or a JBL e1502 for example that would easily deal with the biological aspect of the aquarium in combination with being heavily planted and then add something like the AI Nero 3 (again @wookie planting the seed) which would give ridiculous amounts of extra flow if need, and is fully controllable . Both combined would be around the cost of an 850T, and I wonder if the combination of a decent flow canister and the AI would just take the flow to another level.
For now Im well and truly sitting on the fence still.