I see no reason to single out and dwell on any one specific caliber, unless you are budget bound or the significant other restricts the count of how many you can have (boy, that would stink) If they made a rifle that was perfect for every situation, we would all already have it.
For the folks that think a rifle for a defensive situation is not "doable", take a practical rifle course (typically done with an AR-15 or AK-45 or similar due to ammo cost but not always) from a good instructor....then state that a rifle for defense isn't a good choice. If the instructor delivers the training you paid for, you'll be training from bad-breath distance out to what ever the range supports...hopefully a minimum of 200 yds. While your rifle (or shotgun) may not be where you need it, which would be a problem, that is still no reason to shun it as a defensive weapon. Since you wear a handgun, use it while you navigate/fight your way back to your long gun. Some of us live at the end of long driveways.....hearing/seeing someone approaching doesn't have to be surprise. A good dog worth his keep will make certain of that....a really good dog will fetch it for you.
I hope I never find myself in a situation where I have for the above to be a concern. But if I do, I'll not be running around with just a handgun hoping I had something a lot bigger.
That being said, I have two favorite semi-auto rifles.....and I get to pick which one for what.
FNH FNAR (7.62x51) 10 or 20 round mags
M&P15 (5.56x45)
