I have a Zizzo Liberte which came with 1.5 width tires. After over 3300 miles, the rear needs to be replaced so I purchased a Schwalbe Marathon Plus 1.75 with 3mm protection because I wanted a bit more cushion. The front 1.5 is still good, so it stays for a while. When it is time to replace it with another Marathon Plus, will there be any "real" advantage to match the 1.75 rear tire size? Will a front 1.75 tire have a more noticeble cushion than the 1.5?

Probably no major advantage, just a little more cushion from the 1.75.

Personally, I'd run equal width front and rear unless I was in desperate need of something just to roll on, but a wider tire on the rear is better than the other way round. I'd also forgo the Marathon Plus and instead go for the 100psi regular Marathon. It's a good all-round tye and I've found mine to have extremely good puncture protection. Having a flat doesn't even cross my mind on longer rides far from home. All the peace of mind without the extra weight and rolling resistance of the Plus imo.

Assuming the same PSI, slightly better ride.
If you are precise in your bike fitting, you'll notice the slight tilt forward of the bike.

When you finally replace the front tire, put the older marathon up front.

(I'd replace both and donate the still good tire)
((if you have a bike charity near you))

Why not run a wider tire in the front?

I have seen a fair number of people do this so not sure why not. seems like it would add some suspension effect for a better ride.

I do this with my 650b off-road wheels. I have a 48mm in the front and 42mm in the back (due to more limited clearance). Off-road, maintaining traction at the front is critical, and, as mentioned, wider tire gives more suspension.

A wider tire means greater volume and therefore the ability to run lower pressure (relative to a less voluminous tire). But it also means a taller tire and therefore a slightly higher angle of attack and a slight backward lean to the bike when installed the front. This helps clear obstacles. Coupled with the already lower pressure needed in the front due to the weight balance of a bicycle which puts more weight in the rear, you can run a wider/talker front tire up front with much lower pressure and achieve an overall more plush ride.

The other consideration is traction. Sliding in the rear is a lot more recoverable than sliding in the front which is almost always game-over. A wide tire in the front puts more rubber on the road mitigating slide-outs. Related to traction, a bigger tire in the front will add breaking power as weight is transferred to the front during deceleration and you've got more rubber on the road due to the wider tire.

Finally a bigger, heavier tire in the rear makes pedaling feel a little heavier and acceleration a little more sluggish. So, generally, wider/ taller in the front though a lot of this depends on the surface type also.

what is the model number of the regular marithon tire, if you know it?

The Marathon Plus is wider but surely not more comfortable!

Its a tire with a special thick puncture protection but a very stiff construction.

The Marathon is also quite stiff, it has the same structure but with another thinner puncture protection.

If you want a softer fast tire but still with a very good puncture protection (similar to the Marathon), go for the Continental Contact Urban 42x406.

Unless you are racing hill climb, there is no advantage to run the 1.75 on the back. you would be better fitting you old 1.5 on the back and finish it out and run the 1.75 at the front, the new rubber with the added volume/lower pressure and extra patch contact will give you better control and possibly more wrist comfort.

I run wider front on XC/gravel bikes or at least gripper (50 or 52 width) and of fast race, I run a thinner rear semi slick (47 witdh). On hill climb bike,I run 32 rear and 28 front (for weight reasons)

And if confort is important, conti urban contact is much smoother and faster than marathon.