UpSampling2D is just a simple scaling up of the image by using nearest neighbour or bilinear upsampling, so nothing smart. Advantage is it’s cheap.
Conv2DTranspose is a convolution operation whose kernel is learnt (just like normal conv2d operation) while training your model. Using Conv2DTranspose will also upsample its input but the key difference is the model should learn what is the best upsampling for the job.
EDIT: Link to nice visualization of transposed convolution: https://towardsdatascience.com/types-of-convolutions-in-deep-learning-717013397f4d