Not quite this dilemma, but after reading the research on QoL after bowel resection I was quite keen on having a J-pouch done as part of my ultra-low anterior resection. Two surgeons independently told me my pelvis was too narrow to confidently do it without issues (I’m a reasonably trim 32-34 inch waist guy), and that it only made a real difference to bowel function within the first 12 months anyway (until the remaining bowel distends to create more storage).
I went ahead and had most of my rectum removed (about 3-5cm left) without a J-pouch, and after a few months of some serious adjustments things are now much better than I was warned they could potentially be. If you are keeping some rectum (as it sounds like you are), then good function is still definitely possible - especially with some of the more useful lifestyle adjustments and techniques you can try.
Ultimately if things are intolerably bad you also always have the option of an ileostomy (which I had for 6 months, and was also fine after a bit of an adjustment period).
Anyway, just one man’s experience. Best of luck for the decision and surgery.
10/2018 Dx RC, 12cm high
Mod diff, EMVI+ LVI+. 4 liver mets
pT3N1aM1a Stage IVa. MSS NRAS G13R
CEA: Oct-18= 12; Nov-18= 14, Mar-19= 2.4, Aug-19 <2.0, Mar-20=2.2, May-20=1.9, Jun-20=2.1, Sep-20: 2.1, Dec-20: 2.3
11/18 FOLFOX x6
3/19 Liver resection
5/19 25x pelvic radiation; complete met. response
07/19 ULAR w ileo, 1/27 LN+
08/19 Found liver spot
08/19 FOLFOX x1, FOLFOXIRI x1, FOLFIRI x5
12/19 Liver resection
02/20 Ileo reversed
03/20 NED (CT/PET/MRI) - latest Dec 2020
07/20 Clear scope