The 3841 has an Allen set screw on the outer housing, may be harder to dismount by a perp/vandal than the press-tab on the 2831.
The 3841 has just a tiny bit smaller sensor (1/2.8") than the 2831 (1/2.7").
Neither will have very good low-light performance, an 8MP cam ideally should be at least a 1/1.2" sensor, a 1/1.8" would be marginally acceptable. A larger sensor would be more expensive though.
Looks like a toss-up, IMO...they are spec'd very closely. Maybe an owner will post soon on their experience.