I have a Brother HL-L3290CDW printer with LED lights for the xerographic print system. Before that I had a Lexmark laser 4-colour unit. I do not see any major difference in print quality between laser or LED light sources. As you say, the LED system with an array of fixed LED lights has less mechanical complexity that a laser system that must scan the single beam. Of those two Brother units you specify, both have exactly the same native resolution of 600 dpi (without enhancement by interpolation).
One small difference I see is a bit confusing. For the scanner system, the size of the document glass is said to be 8½ x 11 for both, but then the 8900 model elsewhere says no, it's 8½ x 14 (legal). By the photos the top of that 8900 model seems wider, so the legal size spec seems right. Of course if you are using the document feeder for scanning this does not matter.
Regarding costs of toner, details matter. Each model can use any of three toner cart capacities, called Standard, High Yield, and Super High Yield. But those names mean different things! To focus on the High Yield, the type TN433 carts for the 8900 model are rated for yields of 4500 (B) or 4000 (CMY) pages. For the 3780 model the comparable carts are Type TN229XL rated for 3000 (B) or 2300 (CMY) pages. So comparing those costs means calculating on a per page (or per 1000 pages) cost. By the way, the ACTUAL page yield will not be exactly as spec'd, but you can use those numbers for approximate yield costs.
For years I have not bought OEM toner cartridges. Instead I bought third-party ones by Moustache from a seller in Canada where I live and have been totally happy with them, So I'll use that source's current CANADIAN $ prices for this calculation. OP, you will need to re-do this according to where you get your carts and their pricing. With my pricing data source, a complete set of four TN433 carts (Black, Cyan, Yellow, Magenta, High Yield) for the 8900 model is C$180 to last 4500 to 4000 copies. That's about C$45 per 1000 pages for all colours. For the 3780 model the set of TN 229XL carts is C$260, or C$113 per 1000 pages. But NOTE that this latter toner cart set is WITH CHIP - that is, they have a chip in the cartridge that mimics what a Brother cart would have so they will work with no fiddling! Say, what? MANY printer makers fit their cartridges with chips that identify the cartridge maker and the printer may NOT accept and use third-party units. As it happens, the particular printer model that I have by Brother does this BUT there is a simple process I found on the internet using menus to re-set my printer to accept a third-party cart. Whether that process is possible with YOUR proposed models is not detailed so I cannot tell you.
To avoid that third-party confusion effect, let me re-do those calcs with genuine Brother carts from the same on-line seller. For the 8900 model the set of four type TN433 High Yield carts is C$733, or C$183 per 1000 pages. For the 3780 model the set of four TN229XL carts is C$523, or C$227 per 1000 pages.
So the pattern appears to be that the larger 8900 model that is a higher up-front price has some features better than the other, but also will be less expensive long-term in terms of consumable toner costs. Not by a lot, though. Using the numbers above with genuine Brother carts, it is about C$40 per 1000 pages or US$ 30 per 1000 pages. OP, you can re-do this to get the right number when you figure out your actual consumables cost.
Just a small couple additional comments in toner costs. First, with my Brother printer (and the prior Lexmark unit) the printer driver system for Windows offers an option you must set for EACH print job as it is set up. You can tell it to print in Black only if there is NO colour on the page, and it will NOT waste the three coloured toners on black type. In my Brother system now, there is a very handy tool to create and store profiles of document specs you can apply to each document as it is printed. For example, I have profiles for single- and double-sided printing each in B/W or Colour, and each in Letter or Legal size paper.
Next, the particular toner seller I buy from also sells sets of FIVE toner cartridges - the three colours plus TWO Blacks, since you tend to use up the black faster over time. That can save a bit over buying the four-pack plus a single extra Black cart.