Here's +1 for the IDF Tavor.
I too run a MARS optic on my IDF16. It's THE way to go with this gun. It's a fantastic optic--very clear red dot, plus IR laser which is great.
The IDF model is a collectible, but it also has its advantages. The rear gas block has an integrated BUIS and a QD mount for the Mepro MX3 magnifier, which works with either the M21 or MARS sight. The IDF gas block/integral M21 or MARS also fits on the X95, BTW, if you want to make a more authentic IDF X95 clone. The IDF Tavor is also cooler because it looks almost identical to the military TAVOR-21, which we should all be able to own anyway here in the Land of the Free.
How often are you really going to switch around optics? If you can be happy with the M21 or the MARS, the IDF is the way to go. And the M21 is not a bad sight, as long as it has the triangle reticle. I have one on my X95 and I love it. People hate it because they look through it indoors and it looks tinted and weak. This optic is happy in sunlight. It's a fiber optic sight, after all. Get outside and give it a chance on a bright, sunny day, and you won't have any issues. It's extremely rugged and doesn't need batteries. And it even has tritium. It also looks cool with its green lens and has accessories readily available like a washout reducer and polarizer/killflash kit.
If you want to run an optic other than the M21 or MARS, then no, the IDF is not for you, and why were you considering one in the first place? The Tavor with full length top rail is obviously what you'll need.
P.S. Warning: If you do want to run an integrally mounted MARS sight on a Tavor, you'll have to remove the QD levers from the MARS optic and swap the bolts over to the MARS sight. It took me about an hour to do it right. There are pins you have to press out of the M21 mount and push fit into the MARS optic. Just study the M21 integral IDF mount and you'll figure it out. If anyone decides to go this route and wants more advice, PM me and I'll get you some pics so you know what you need to do.
My MARS IDF Tavor is my favorite gun.
At least until IWI finally releases the round handguard for the X95... then that will reign supreme.