Before we get into suggestions, I've got a question: You already have a Mossberg 12 GA slug barrel. What do you want to do differently with this Handi-Rifle?
I think that ought to be settled before you place your order. Is this just a back-up gun? Is there something you don't like about the Mossy?
Me? I had a Rem 1100 with a plain rifle-sighted slug barrel for years, and then I bought my Mossberg 12 GA pump. A year later Walmart had a rifled slug barrel for the Mossberg that was ridiculously mis-marked, and it was just too good to pass up-- I even went to the manager and asked him flat out if he would sell me the barrel at that price and he looked at me like I had two heads. To be truthful, I haven't hunted with that slug barrel more than a few times. It does give me a longer reach, but when I use the Rem 1100 I'm usually in dense cedar thickets, and I'm taking jump-shots at 20 yards-- a scoped long-barreled pump just isn't the right medicine. Do I regret the purchase? No. I'm just saying.
Back to your situation. The Handi-Rifle may give you better accuracy-- don't know for sure. I don't know how good your 12 GA is. It will certainly give you lighter weight. If you want a lightweight gun, definitely stick to 20 GA. Lightweight 12 GA guns are punishing. Scope? That depends on your eyes and your range. I've gone to about 90% scoped long guns because my eyes are going bad. If I had my choice, I'd probably have about 50-50 iron sights. If you're talking mostly 50-60 yard shots on the top end, I'd stick with irons as long as my eyes would let me.
BTW: I always try to answer "WHY?" before I do any gun purchase, but "Just cuz" is always a good reason.