Personally, and this is just my opinion. I thought declaring transactions at your service/use case level is the place to do it, and I personally would use @Transactional, but that is my preference. Rather than manually code transaction support, which couples it with that code. Which we want to avoid tight coupling, otherwise we wouldn't be using Spring.
If you don't like @Transactional, then what about declaring Transactions through Spring AOP. Then it is 100% decoupled from your code.
Mark