As both a product manager working in payments and a steem fan, I'm super keen to see this! My thoughts:
Anything with "steempayments" in it, or something else wordy has the benefit of being easier trade marked. The simple "Pay" buttons, even with the logo have very low chance of getting a trade mark (you generally will not get a trade mark on a common word or a word describing the action/activity of the product).

Does Steem have a brand guideline or style guide? I ask as I note you are using the light aqua blue for the some of the text, which is what Steemit uses as a sub-text (ie the beta in steemit beta). The call to action is infact paying, its not something that should look like a sub text.

Other musings.. you should do a small screen breakpoint mobile version too. Thankfully steem has a logo thats already reasonably mobile friendly.

Finally, in terms of brand heirarchy- where do Steem, Steemit, Steem Payments, Steemd sit and relate to each other? This may already be done, but before expanding out to sub brands we really should all get on the same page regarding "branded house" vs "house of brands" as this is unclear to me, even as an existing user.