There is one thing you may not realize you're doing when you knowingly use an affiliate link. If you sign up, or enter any useful information at all, you are providing statistical data that will forever associate you with the sort of person who responds to this type of lead generation. While it's debatable that this is harmful in any way aside the nuisance of targeted media, it will almost certainly cause a stream of advertising in similar formats for various other offers, which could in turn lead you to sign up for things you don't need.
Affiliate links are simply a way to channel conversions, log specific data, and apply incentives. If you go through the bank itself, the affiliate misses out on a commission, but you still get logged as the sort of person who signs up directly and thus, a different kind of targeted media.
There's pretty much no way to avoid being a statistic in this game. If you get approved, every purchase you make is data they can use to narrow down the targeted media. Understanding that and accepting it is part of the contract when building credit, and is not necessarily anything to be afraid of.
In the end, as the consumer, the finalized product is identical unless the affiliate or direct method outright advertises something special to incentivize you.