Splitwise user who absolutely loves it here. I wouldn't think to do budgeting on it at all. Its purpose is splitting expenses, nothing more. It does it great and the single function is a part of why I use it. Hooking up with Venmo, big plus. People here are talking modularity, and those two services are great examples.
Honeydue looks to be budgeting and larger finance management targeted at couples. I don't know Buxfer well, but it looks like the couples targeting is the variation it has over it.
As someone in a long term relationship who expects to be sharing finances relatively soon, Honeydue looks like a very interesting option. Currently we are separate but discuss and keep tabs on spending together, and have one giant splitwise going. The majority of that budgeting is done on Excel / Sheets - good for customizability, but a bit of a pain. Once we merge accounts and the like, that Splitwise goes away and Honeydue may just come in now that I know about it. It if was just me, I would have no need for Buxfer.