Depends on what you want to carry in it/use it for. I think the TPM would be a great little cross-body when you're travelling and just want easy access to your boarding tickets and passport etc. Or I'd toss it over my shoulder for a quick beer out where I just need to take my phone, readers, and a card case. If you need to carry reading glasses and sunglasses, and/or a larger wallet, I'd go for the larger size.
Do they currently make the herbag in the TPM size? I feel like I've seen them in the resale market, but have yet to see a new one... Would love to get a new one if they are coming back since I love smaller bags!