I use the plastic one whenever the amount is under $100 as that allows me to just tap the card and pay. The Titanium card doesn't have the tap feature.

We also have many of these Amex promotions, spend $X at retailer Y and get $Z back (e.g. spend $15 at McDonalds and get $5 back ). I usually register both Titanium and plastic cards for these offers and then use both if the offer is useful.