I bought 1 cot and 1 playpen.
I agree that the playpen can be a bit shaky when the baby is sleeping. To prevent baby's feet from sticking out of the cot, you can use a cot bumper. But the thing with cot bumpers is that it can cause the cot to be hotter than normal. This wouldn't be a problem if your baby sleeps in air con.
I used the play pen for my boy when he is awake or napping. I put toys in it for him to play with but I only used it for a short while before he starts being unhappy being in a confined space. So I only used my playpen for 6 mths.
My friend's kid learnt to stand in the playpen and he was so frustrated he stuck his leg through the bottom of the playpen.
My baby is 17 mths and still sleeping in his cot. My cot is convertible to a toddler bed which I hope to use till 4yo.