Here is my take on SCUBA gear when it comes to brands. I think there are a handful of companies that will manufacture certain equipment items for different brands. BCD's come to mind, although some companies will manufacture their own branded BCDs, I think the majority of the mainstream and value brands are built by the same company or companies using the same methods and materials. So the main difference there is comfort, aesthetics and of course price. I believe the same to be for masks, fins, snorkels, and accessories.
I am not completely sure about regulators. I think that the major brands that have been around a long time like Scubapro, Mares, Dacor, US Divers etc... play a larger part in the manufacturing process of their regulators, perhaps some even have their own manufacturing plants solely for their brands. However, I don't see how it is economically feasible for the value brands to do the same, so in that category the few actual plants that rebrand the regulators is probably more common.
Local dive shops, (LDS) will more times than not push heavily the gear that they sell, some might even tell you that theirs is the only brand you should get for XYZ reasons. The shops make money from selling gear, so it is natural that they will try to sell you theirs, I do agree that a shop should not talk bad about the gear they don't sell, but it is up to you to decide to give them their business or not. You need to be aware of this when asking a LDS owner, employee or affiliated instructor.
When buying gear as a new diver, you need to consider first what environment and conditions you will be diving in. Second, before you make a huge investment, make sure that you will remain an active diver, and then buy the appropriate gear. Third, make sure that the brands you are buying will be serviceable in other regions, not just locally. In my opinion, if you are doing less than 10 dives per year you should rent your gear, as it will be more economically feasible.
Oceanic has been making SCUBA gear for a long time and have good products, they may not be the best, but certainly not what I consider a value brand. With proper care, storage and service the equipment will last you decades. I honestly am not familiar with SEAC so I have no useful opinion there.