Typically
dive fins are open-heel - it's the optimal configuration generally. Typical scenario on a dive boat - you gear up, stand up, walk to the back, grab the rail and put each fin on in turn before splashing. Some boats won't allow you to walk on the deck with your fins already on - it's not safe. When you're reboarding in anything but the most benign conditions you typically float on a line off the back of the boat, take your fins off, slip them over your arm and then go up the ladder - or hand them up. The straps make both of those operations much more convenient.
For shore diving, you'll be taught to put your gear on and walk out to deeper water before putting your fins on. In some areas good thick soled treaded boots are a requirement also - just off the shore line there can be sea urchins, sharp coral, ironshore (fossilized sharp coral) and other hazards. Go to Bonaire (shore dive capital of the Caribbean) with full foot fins and you'll come back with shredded feet - it happens occasionally even with boots.
Most open-heel fins have enough adjustment so that you can wear either thin boots in warm climates or thick boots when it's cold. A popular option is bungee or spring straps they're much less likely to break. Compare them to standard fin straps in the store - I break one about every 4-5 years - they just can't take the abuse - or often the exposure to the sun.
Full foot fins have a following. It's people who only shore dive off sandy beaches or boat dive on small boats where they often put fins on then backroll off the side of the boat. They're also very popular in rental fleets since they don't also have to stock rental boots.
Split Fins are optimal for one thing - going forward fast with less effort. And they're nice if you're old/when you get old. But you can't back up in them, do fancy turns etc. since the very design means they split when retracted. They're also generally more expensive because the designer locked down his patents tight and charges a hefty royalty per pair - to this day. The reason you see full-foot split fins for a 3rd the cost is because he missed that.
A good pair of paddle fins is >$100. Since you seem to like Mares, let me tell you from experience that their locking fins straps are one of the worse things I've ever used. I bent back several nails trying to get them to unlock before I threw them away and went back to standard straps.
Are you looking for lightweight for travel purposes? Because the Mares Hybrid isn't what I'd call a lightweight BCD at 6.2 lbs. Even a Zeagle Stiletto - which functions as both a cold eater and warm water/travel BCD is 5.7lbs.
If you plan to travel to dive, get a travel optimized BCD instead - something like a Zeagle Covert (under 4lbs.) Aqualung Zuma or ScubaPro Lighthawk. Just about every mfr. makes one also. One that's sort of caught my attention lately is the Hollis Ride also - they seem to be directly competing with Zeagle's Express Tech - a minimalist soft backplate design. It will actually take doubles (not very well) if you ever decide to. Another nice feature of travel BC's is they fold up/roll-up - pack small. You can carry on a Covert, it rolls into a big newspaper sized bundle. The Zuma comes with a drawstring bag for the same reason.
I also wouldn't buy expensive gear before certification. And understand that your dealer may have ulterior motives in suggesting that.
First of all, most of them get
one good shot at selling you a lot of expensive gear - during your class. Some also have marketing agreements to fulfill so that may bias their suggestions. In addition, many times your instructor will be outfitted in gear supplied by the shop during your class - even though his optimized personal gear is completely different. Coincidence that it's generally not the lowest end stuff they sell? Occasionally an ad pops up in our classifieds from a dive professional selling his personal gear since he's doing too many classes to use it. There's also a keyman discount offered by some mfr's to instructors and others in the industry who are regularly seen in gear.
Buy your gear, use it in the pool and find out it's not right for you? Your shop will likely take it back - as
used. Likely for 1/2 of what you paid 2 weeks earlier.
There's a shop in SoCal where they oversell you on custom stuff - including several thousand dollar drysuits - in SoCal that makes some sense. But "your" custom drysuit is not returnable. And neglect to mention that trying it on and getting it wet in their pool makes it used....