Well, I actually think there is a big obvious design difference between the two. It looks like Fluke's designers had the user in their minds, whereas Brymen didn't think much at user's experience.
I don't have any of these two meters, but I always have a Fluke 87V and a BM789 on my bench, and try really hard to like BM789. I like it, but can't love it.
The Flukes are designed (ergonomically) to be actually used. BM789 seems to be designed to be packed with extra-features (one might or might not need), thrown together to impress on paper. They do deliver on what they promise, though.
When you get to use the BM789, you figure you have to press 6 (that's six) times the yellow button in order to switch from AC mV to DC mV (you have to circle through options you might not even need in the first place). Every time you switch the rotary dial, you also have to double-tripple check if you are on the right function (ohms? buzzer? conductivity?). Like other handheld meters that don't care about ergonomics, it always shows a variable number of useless zeros at the left of the decimal point (006.67 mV instead of 6.67 mV - you have to look really well to make sure you don't misread the position of the decimal point). I know some people don't mind, but many do. Also, look at the rotary dial of 77 vs BM786: the positions are nicely spaced, easily readable on the Fluke, crumbled together and hard to read on the Brymen. These might seem minor things, but it is obvious Fluke did think about them, whereas Brymen didn't. Other than the ergonomics, BM789 is a great meter.
If you don't need the extra features and precision of BM789 (i.e. if you have other specialized meters for that when you need them), there is really no reason to choose one for everyday use. I always tend to grab the Fluke first, then sometime refrain myself and use the BM789, to impress myself with its accuracy (and ask myself what they were thinking when they designed that user interface...).
On the other side, Flukes seem stripped down to the bare minimum for their intended user (87V is an exception). They have an 88V for automotive use, which is pretty much an 87V to which they added RPM but removed the true RMS (why?!) and the high res mode (if I'm not wrong). Or they sell an 116 with a uA position (for flame sensor check, if you are one of those guys who service furnaces) but no Amps (you have to buy the 117 if you are an HVAC technician who cares about Amps but doesn't work on gas furnaces, because you don't get uA on 117...). You really have to know who you are and accept it upfront. Ridiculous.
Fluke 87V (and maybe Fluke 77, too) are nicer, faster and easier to use (... and outrageously expensive, especially outside US - as it seems). BM789 (and 786, I guess) are the way to go if you only plan to have one quality reasonably priced meter, not sure what features you might need in the future (dBm? DC+AC mV? two temperatures? 4-20 mA? the highest counts/$ you can get?) and don't mind working harder to make a simple measurement.
Not sure if the ergonomics is factored explicitly in the price (e.g. if Fluke paid an arm and a leg to the design guru and it would take them decades to recover the investment), but people do like ergonomics and if they are willing to pay for it, the prices stay up. As in any other product, the fair market price is what people are willing to pay, not the price of the materials and manufacturing plus a reasonable margin to keep the business going.