Are broadly similar. The Advent has additional USB port, which can be useful, plus a better webcam. Confess I have no experience of Advent, as a company, so you would be well advised to check out some user reviews, to ensure there are no "common issues". Both would appear perfectly useable, for routine "stuff", provided you are not looking for gaming capability, or something like video editing.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been