Hi Twinkle (and as I’ve not seen you about here before…Welcome)
I personally don’t like panniers as they are too specific to bikes (I guess that's obvious). If I am going to spend cash I like to know that I may at least get more than one function from an item and so I opt for a rucksack every time.
The trick with a rucksack is to look for one with a really good, comfortable venting system on the back, the bit that sits on your back. I have a "Vaude Bike Alpin Air". it has a "frame" on the back that has a mesh panel stretched between the two sides of the frame, the mesh sits on your back and cool air flows between your back, and the rucksack. It proved perfect all summer and provided a great tool for walks in the park and picnics with the kids etc. It also looks pretty smart (rather than a little bag perched up on your shoulders)
I love my rucksack and would really recommend the "Air" system (or the brand). The alpin air comes in a few different sizes (I have the 30+5), is really adjustable (even to your torso height/size) with plenty of adjusters to enable the right fit. If you get one, do read the manual (that, coming from a bloke!!) as it explains how best to set the bag up for max comfort. The bag has tons of compartments for dirty washing, clean clothes, wet weather gear, repair kits, fleeces, Lights etc and it also has a few “secret” compartments for cash, jewellery (cufflinks for me) and the like. Best of all, it has a handy orange outer that you pull out of a bottom pouch when it rains, to keep everything perfectly dry (and this really works well)
Good luck with the commute, Hope you find what you’re looking for.
Jonny