Just compare both model, look at the weakness and strength, and what you are looking for that fits your need and budget. Example if your want best image you have to go with 4K, if you don't care about that then 1080P will do just fine. The difference between the two that you mentioned is not a lot. The 2100 has 2500 lumens and a contrast ration of 35000:1 compare to the 880 which is 3300 lumens and a 15000:1. That being said with the 2100 you'll get slightly better image but the 880 is brighter. Also note that the 2100 is slightly bigger and heavier that the 880 and possibly a little more expensive and I believe is also a discontinued projector. So in my opinion I'll go with the 880 which may be cheaper and is a newer model, check my review and photos on the BestBuy website .
Hope this help.