I have an Oasis but I'm probably going to upgrade to a Tandem Island, primarily for the sailing ability but I'm also looking forward to seeing how much better it'll be in kayak mode.
The primary advantage should be a significant reduction in wave drag due to the longer and narrower hull and since that seems to be a large drag component for the Oasis at normal cruising speeds the Tandem Island should be faster although it's difficult to say by how much.
The other big advantage, apart from more storage space as you mentioned, seems to be a more spacious front seat. The Oasis is somewhat cramped in that department and if you and your friend haven't tested one to see if it's large enough, that could be a showstopper right there. The dual steering controls also seems like a nice feature on longer trips since you can take turns steering while underway, might be a problem if you can't agree on where you're going though.
Apart from the much higher price the only real disadvantage should be the increase in length and weight but It should be manageable since you are two. The stability might also be somewhat lower due to the narrower hull but I doubt it'll be a problem since the Oasis has plenty of stability and the Tandem Island is still 6 cm wider then the Adventure whose stability I haven't seen anyone complain about.