Since the idea here is to retrieve Accounts, it should be inAccountService.
Both methods looks fine in AccountService to me because you're operating on accounts. If you want to persist the transaction you could have a TransactionDao handling this for you and you'd be calling it from your AccountService each time you need it. Doing both in your AccountService methods will allow you to be transactional. You don't want to persist the transaction object if updating the balance raised an exception.
Services are useful when you have business logic which does not belong in your DAO layer. The latter is supposed to query your database and give you back appropriate domain objects whereas services are mostly used to do additional treatments like handling transactions or DTO mapping.
You should take a look at the official Spring sample app PetClinic to give you an idea.
