Side and Icmeler are totally different.
Side is a lovely old town with Roman ruins and most of the hotels are on the outskirts.The beaches are sandy here.It is very popular with other Europeans and British are in the minority.I first when to Side in May 1994,then went back 3 years ago.It had not changed but it is more expensive than Icmeler.You have to fly to Antalya to go to Side and there is usually less choice of flights than Dalaman.
Side is very flat with no islands out to sea and the mountains are a fair distance away.
Icmerer has grown from a small village to a lovely resort.There are no Roman ruins in the town but it is set on a lovely scenic bay.
It is popular with British,Dutch and Germans.Nearly all the hotels and apartments are in the town itself and on the sea front apart from those in the old village which is a short walk away.The bay looks out onto islands and the resort is surrounded by pine covered hills.
There is a small canal with shops and restaurants on either side.Its lovely at night when its lit up.
It is very reasonable to eat out here as there is a vast amount of excellent restaurants to choose from.
Their are more flights to Dalaman than Antalya from the UK and I think Easyjet now go here from Gatwick.