We have owned both the Oasis and Tandem island models, they are both nice boats and you can't go wrong with either.
A lot also depends on where you live also, if you live near the ocean and plan to go offshore on in the intercoastal system, you will find out quickly that these bodies of water are huge.
Let me go into how we got into kayaking and the choices we made, this may help. When we finally moved back to Florida for good and sold all our other houses, we had already sold our Sea Ray 24 ft boat a few yrs before. We always loved the water but no longer owned a boat so we decided to take up kayaking. Our first few times out were in rental paddle kayaks where we were only comfortable a few hundred feet from inland shore (on the intercoastal in Florida), we would venture at best a mile or two from launch, then after an hour or so we were usually tired and would return. We then went to a local Hobie Dealer Demo days event where they let us try out 5 or 6 different models of kayaks (all brands), they were very knowledgeable and tried to guide is in a direction that would best suit us and answered all of our stupid beginner questions.
We ended up buying a revolution for my wife and an Oasis for me. We have 6 kids (all grown up now) and 6 grand kids who come down from Illinois often (to get out of the snow) so we wanted a two seater that can handle two people easily yet still usable as a single, the Oasis was perfect, but when solo the Revo was way faster (Revo's are very fast kayaks), and my wife would literally drive circles around me taunting me to keep up (true story). We probably put a couple hundred thousand road miles on our car with kayaks on the roof and camper in tow traveling around the country dropping in any body of water we could fit the boats into (including class II rapids in Colorado). We had the optional sail kits mounted on the boats and never went out without the sail kits strapped to the side of the boats, just in case we got a chance to sail. After a while we could go 10-15 even 20 miles with little effort pedaling 10 hrs, then get up the next day and do it all over again. That's way further and way less work than we ever imagined when we were renting paddle kayak, didn't even know it was possible.
In 2010 when the Tandem Island came out we traded in our Oasis for the new Tandem island, and continued on our merry way doing exactly the same stuff we were doing before. I always load the kayak onto the car all by myself sometimes 5-6 times a week. Yes the TI is a big boat, but the Oasis is also large, personally I prefer to load a TI on my roof over our old Oasis any day. It's about 3 ft longer, this in my opinion makes it easier to lift only one end (you only are ever lifting around 50 lbs on the TI or 45 lbs with the Oasis, I honestly cannot tell the difference.
One reason we got rid of the Oasis was it had really limited cargo space, we are divers and campers, and always have a lot of gear on our sometimes very long excursions, there is no comparison as far as storage space goes, the TI has way more. Also the TI is by far the fastest boat in Hobies fleet, even solo in kayak mode I could easily smoke my wife and would constantly drive circles around her taunting her to keep up, well until I tried to do it on a very narrow river and got sideways right in front of her and ran into the bank and we both went over (I'll never live that one down). We were out last weekend at a Hobie Island club get together and one of the guys had his whole family on a TI, he had one of his kids I would guess around 6-8 that just rode on the huge rear deck behind the rear seat, she actually looked pretty comfortable back there. We have had 3 adult on our TI in kayak mode and as many as 4 adults and two children on our TI (in TI mode with the sail, AMA's and trampolines) out in the gulf on nice calm days to go out snorkeling and having fun down in the keys. Actually we are out on it ever single weekend all year round. We are leaving for our other house in Key West in the morning for a week or so, and I plan to go out ever single day if possible. We will be going out with 5 or 6 people probably every day. We have an inflatable 6 person boat that we just blow up and tow behind the TI, we store all of our coolers, scuba tanks, swim gear, etc in the towed boat about 20 ft behind the TI (you can't even tell it is back there), we also have several cheap inflatable single person kayaks, that people just grab onto the sides of the towed raft and we pull the whole group on our 'party barge' off shore somewhere or at a local island or sand bar and just have fun snorkeling and cooking out.
The TI in my opinion is the ultimate family boat, here is one of my favorite TI videos of all time.
I'm not trying to influence you one way or another, either choice is a good choice is good, as they are both great boats, but Hobie literally invented a whole new category of family boats with the TI, you can do more with a TI than any other boat on the market today, it's truly the family SUV of the boating world, and in a class all by itself. You can literally use it for anything you can imagine.
I definitely would demo both boats and talk to the dealer at length.