I bought my bag gloves and my sparring gloves at the same time. They are both the same weight and the same brand. The bag gloves are made out of layered foam while my sparring gloves are (or were ) made out of moulded foam. I was told that layered foam gloves will last longer and be better for bag work while moulded foam gloves wear out faster but are safer for your opponents in sparring.

So despite the bag gloves being supposed to last longer they are nearly dead and I wouldn't have to buy new sparring gloves at all if I hadn't lost them (stupid mistake!!!! ). I am using the bag gloves now for very light sparring until I get new sparring gloves but I need to be ultra careful because I don't think they're safe at all.

So yeah I agree with your training partners that it's best if you can save your sparring gloves for sparring and bag gloves for the bags. Even get two pairs the same if you like them so much and use one pair only for sparring and the other for bag work and see how much faster the bag gloves wear out and how much safer the other ones are for sparring with.