Marinating in a bag vs pan
The main difference you'll notice is that if you use a pan you will need a lot more marinade liquid to submerge the food.

Using a different amount of marinade could have slightly different results, particularly in regard to the absorption of salt. Meat will only absorb salt until the salinity inside and outside of the meat reach equilibrium. Using less marinade would mean this happens sooner. However, that really only matters in brines rather than marinades because 1) it takes a long time and you probably shouldn't marinate for that long, and 2) your marinade recipe likely doesn't have enough salt for it to matter in a way you'd notice either way.