Hello -- Thanks for your question. The answer depends a little on what you mean by "firmer" and also on what position(s) you commonly sleep in.

If, by "firmer" you simply mean "supportive," but you sleep on your side and/or prefer to sink into the bed a bit, then a pillow top could be a good choice.

On the other hand, if by "firmer" you mean "not soft" AND you generally sleep on your stomach or even your back, then there isn't much point in getting a pillow top. The downside of a pillow top is that there is more that can go wrong from a durability standpoint. The weak links in a mattress are almost always in the upper padding layers, and a pillow top basically doubles down on these culprits. Unless you stand to benefit from the softness, cradling or pressure relief that a pillow top has to offer, there isn't much point in taking the risk of it developing problems (eg, body impressions) down the line.

As for buying a mattress at Costco... The pros are that they have great service in the event you need to return it, and they tend to have good prices. The cons are that it's very hard to choose the right mattress there, since they typically have only 1 or 2 choices (any given bed will only be the right fit for a small minority of people), it's very hard to really try the bed in the store, and there's no one with any expertise there that can help you determine whether this bed is a good fit for your needs and preferences.