Paralletes are longer and therefor will have much better stability. This is especially important for Handstands and planches when your legs are not in contact with the ground and your body is entirely supported by the bars.

In addition, parallettes will have a larger diameter so the grip will also be more comfortable. This will also be more important for L-sits, handstands and planches when your entire weight will be supported by your hands,

Pushup bars are smaller and will therefore be much lighter and easier to fit into your bag. In addition many pushup bars are made of plastic where as parrallettes are made of steel.

Pushup bars are generally made to be just for push-ups and the height will be around 10-15cm from the ground. Paralletes on the other hand can vary between 10cm-40cm. The 40cm are generally ideal for those that want to be able to do all exercises without limiting the range of motion.

The low versions are better for those that want to focus on handstands and planches and will also be easier to transport. Also, the low variation will generally be more stable.

The higher paralletes will also require a block to increase the height of your legs to match the bars when trying to perform a pushup.

Due to the smaller size, the push-up bars are cheaper to manufacture and ship and therefor will cost less. You will most likely pay around 3x more for parallettes depending on the quality and size.