seachem fusion alk part concentration: 4400meg/ml
seachem reef carbonate concentration: 4000meq/ml
same stuff otherwise
seachem fusion calcium part concentration: 100,000mg/ml
seachem reef complete concentration: 160,000mg/ml
same stuff otherwise (at least the important stuff: calcium,mg and sr in balanced proportions. the other 100+ elelements completely unncessary in a 2 part, untestable by hobbiest and thus unbalancable not to mention being altered by food, waterchanges and everything else added to the tank). In fact I have been using reef complete with reef carbonate as my 2 part for years (and reef complete with pro buffer dKH before that). Now its going to change to Fusion alk part and reef complete based on the higher concentration of the fusion. I could care less about dosing in equal parts when the end result is the same even if I have to dose two or three times the amount of alk part to calcium part which is typically what I have to do now using what I use.
I'd chose the fusion over ANY two part based on concentration. you will use way less of this than almost every 2 part on the market. All two parts have watered down calcium parts so they can be dosed in equal parts to the alk part. this is why I dont use 2 parts, they dont charge any less even though they are typically 1.5-4x less concetrated than reef complete which also contains mg and sr in balanced proportions(or kent liquid calcium or brightwell calcion or any stand alone calcium chloride product). based on the concentrations you'd run thru a few bottles of the kent 2 part and most others before going thru a same size bottle of Fusion 2 part. unless the kent is significantly cheaper I'd use the fusion.
beyond that its just a matter of using who you trust to use quality raw materials and well proportioned blending. you'll have no way of knowing this. alk parts are always just a blend of baking soda, baked baking soda to raise pH and sometimes borate to artificially raise hardess (but not carbonate hardness which test cant tell the difference so I dont like). they REALLY arent to be concerned with from brand to brand. the calcium part is where there's going to be an inadequecy if there is one (and where all the other trace elements are mixed with) but again you have no way of knowing beyond reputation. neither seachem or kent have any notable defficiencies in this regard but seachem products are almost always more concentrated for the same price or less then kent.
as for brightwell I find their products to be virtually identicle to kents just with slighting higher concentrations (should be no suprise based on brightwell going from that company to his own). example: brightwell restore = kent coral accell. same ingredients slightly different concentration. I can find brightwell/kent equavalents for half of their products and I seriously doubt he went and seeked all new suppliers vs links he already had with kent in starting his own company. kent = brightwell lite.
