Which suitcase for this trip?

If you can manage the 24 then I’d go with that. Can you lift it, possibly carry it upstairs, and otherwise handle it yourself? If so then I’d take it - but NOT use it to add a ton more stuff, just to have a more space for a still-limited amount of stuff. The struggling to re-pack an overstuffed suitcase is very stressful to me. Better to have a bigger case with room, especially since you are checking it anyway.
The issue is the underseat rolling bag. Can you take a smaller underseat bag that could fit inside the suitcase when on trains?

Thanks for your reponse, Mira. I haven't shifted everything over to the bigger case yet so I don't lnow about lifting it; however it does have a side handle which the cabin one doesn't have.

The rolling under-the-seat one is a new-ish purchase so I I haven't used it on a flight before. I bought it after my last trip as my "handbag" was filled with heavy items to keep my cabin bag under 7kg; with recent back strain, I had found it hard to carry along all those endless airport corridors. The rolling under-the-seat one does have a deep flap attached to fit over the handle of my bigger case. If it does prove really awkward, I'll just have to ditch it at some stage (give to my sister or an op shop?) and buy a cheap capacious handbag to use instead.

Bending over, pushing and straining to squeeze everything in, especially when re-packing later in the trip, is what I find hardest, hence the wish to have more space within the larger case.

Edited to add: re-reading your reply has given me a new idea. Take the old handbag for use on the plane, but keep it much lighter, and leave enough room in the suitcase to stash it in there when on trains.

7 kg. is just about impossible for a 21" carry on bag as most bags weigh 6-8 lbs. empty. I find my bag consistently at 10-11 kg when packed moderately.
Everywhere we go, I see husbands hauling around one or two 20 kg. 24" suitcases through airports, and all I think is "those poor guys."
We have found that it's not difficult to travel indefinitely with a 21" carry on bag, especially if you have access to a laundry to wash your clothes. On our recent trip, we returned home with all clean clothes and didn't end up wearing half what we carried. We intend on future trips to reduce the amount of clothes we carry.

Have you considered a highly water-resistant jacket with lots of pockets in addition to your carry-on and personal item? The company advertises it as being like another carry-on. Also, wear your heaviest clothing until after your bag has been weighed. This forum has lots of ideas for light packing in your situation, such as a puffer jacket that packs to almost no space and I'm a fan of silk long underwear-takes up very little space. Can you limit shoes to two-wear a waterproof pair and pack ballets or mary janes that don't take up much room?

7 kg. is just about impossible for a 21" carry on bag as most bags weigh 6-8 lbs. empty.

This statement is untrue.

My lightest 21” travel pack weighs 2lb. My heaviest 21” pack weighs 4lb. Several rolling bags come in at 5lb. The IT carry on, for example, weighs in at 3.8lb.

I never check luggage, but in your case, I would take the 24”. I was able to pass the 7kg requirement last year with my Eddie Bauer Expedition roller bag & LLBean daypack flying to Cambodia, but I think you will need more clothing/shoes than I packed.

Just because you have extra room, you do not want to go carte blanche with the weight. Try to select the lightest weight items, and make a packing list that you adhere to the day you pack. Enjoy your trip!

Thankyou, Jean. 24" suitcase now packed and ready to go - 15 kg (= 33lbs ?), and that includes my Gortex-lined boots & dressy shoes. Shall wear the more comfortable walking shoes on the plane. Non-wheeled carry-on "handbag" is now a manageable weight. No to the multi-pocketed jacket idea: I'm too short & bulgy already, and in any case resigned to checking in this time.

Yup, plus if a suitcase is really packed to capacity, that's putting extra strain on zippers, and you don't want that rupture happening. Glad you went w the bigger bag, have a great trip!!