I have both. The bed in my bedroom has a box spring and about a 15" mattress. I am about 5'1". I have to climb onto the bed.
On the platform bed I can easily just get on the bed. The mattress on the platform bed is a pillowtop mattress. It is in our guestroom.
Both beds seem firm. But the mattress in the bedroom is new and firm. So both beds are firm but comfortable. In fact I had a guest stay for a few days last month. That was his first comment when he got up. He commented how comfortable the bed was and he liked it. Funny, he has slept on that bed frequently.
I prefer using the platform bed just because it is lower and easier to get on the bed. But both beds seem to be very comfortable for me.