The Beauty Blender retails for about $20. Sometimes you can find it up to 20% off, the Real Techniques Miracle Complexion Sponge retails for $5.99 and I picked mine up for 50% off when their website was having a sale. Right off the bat price wise the Real Techniques wins on price.
Often times when you try a store or generic beauty blender you do not get the same quality, finish of consistency. The are either too dense and hard or they pick too much product. When damp, the Real Techniques Miracle Complexion Sponge is the closest dupe to the Beauty Blender in softness and the finish that it gives you. Most of the cheaper alternatives are too hard and don't feel the same. They come in tie for quality and finish.
I actually like the shape of the Miracle Complexion Sponge, the flat surface is really useful. I would love to hear if you agree with me if you've tried both as well.