I have been aware of SDS's existent for well over a decade because of my involvement with another industry (automotive engine conversions). They have been selling EFI for a long time, successfully. I don't think they have changed their website in that decade. LOL.
I have absolutely nothing against EFII or its owner. I am sure I could be just as happy with his system as SDS's. The two companies' systems have vastly more in common with each other than they have differences. Everything but the ECU's are off the shelf automotive parts, with some custom mounting.
The big difference I see is that it looks like EFII chose a more cookie cutter approach, and they do more of the installation for you. More of the wiring and connectors are pre-made, which might be a great idea if the buy is not VERY comfortable soldering and crimping wires. The downside, if you consider it a downside, is by putting extra connectors at the firewall, you create more potential failure points in the wiring. Extra connectors, professionally done, might be more reliable than fewer connectors done by someone without a lot of wiring experience. Look at your wiring ability, and make your choice.
I chose the "cookie cutter" approach with my engine conversions, but it was an easy choice as the install vehicles were always the same. With homebuilt aircraft it is a lot different. Firewall forward they might be pretty similarly laid out, but that is about it.
Like I said I don't have a great preference for either. I have a few things I would rather see on both systems, mostly from a wiring standpoint. But I would start out using either system, bone stock, as they both work.
I have emailed Ross at SDS extensively, and have found him extremely knowledgeable and helpful. I would guess the owner of EFII is similar, based on what I read. I will be buying a system in the next month or 3, and I will be buying SDS.
But I would not disparage EFII for their different product philosophy either.