Couple o' thoughts:
1) If you get on first/early who cares if you put your bag sideways in the overhead
2) I've never thought "Gee, I wish I brought a smaller bag"
3) Running shoes take up so much space, you'll need the extra 2" -- or maybe it's just my big size 11 feet
4) Is the Hartmann OK to check on the way back (if desired)? i.e. I have a really awesome *read expensive*Mulholland Brothers rollaboard and had to check it once in EDI. Fearful of it being ruined/lost/stolen down below.
5) Clothes get less wrinkled if they're not crushed into the smaller bag.
Wow, now I want to go shopping. (Like I need another travel bag/suitcase -- but it's not as if one bag is exactly right for every trip).