Two Small Suitcases or One Medium - Rental Car Most of the Way

It doesn't matter......

Doesn’t matter as long as you can hide them in the boot/trunk of the car.

Only with regards to carrying onto the airplane. For the car, no.

Depends on the size of the rental car. 21 inch rolling bags work best.

Have you considered using Rick Steve's packing list? My wife and I use it religously, and we never even wear all the clothes on our 2+ week trips.
I remember my poor father having to haul around my mother's 45 pound suitcase all over the world. I swore off hauling unneeded items.

as regards to suitcases in cars check with the rental company as some of the cars have a pretty small boot space (trunk to you Americans).Larger family cars should be fine but any compact cars you might find a struggle.

Thank you all for the advice! The car we're renting is a bit small, so perhaps we'll go with two littles. I appreciate your input!

It doesn’t matter except when you check into a B&B or hotel, get a room on the 3rd floor, and there is no elevator. 2-21” suitcase should fit into all but maybe the smallest of vehicles. I’ll lobby for the smaller suitcases

For cars remember here a VW Golf, Ford Focus is a family sized car. Most average luggage will fit.

I vote for 2. Either his and hers. Or for Scotland in March Winter or Spring. We drove a small Renault and 2 bags fit nicely in the boot. Plus Cameras and Day Packs.

You're all lovely for responding! I think we'll go with two littles to make sure we can fit them in the small rental. Thank you all again!