is it easier to transfer to a top 20 school or to apply as a first-year student?

First things first those schools are not top 20 schools. Austin is a T40 school and SMU is a T75 School.

They both have a first-year admit rate between 40-45% but UTAustin's transfer rate is about 33% and SMU's transfer rate is about 50%. So it's slightly easier to transfer into SMU.

If you want to check the difference between first-year rates and transfer rates just google search the "common data set" for the school in question. Section C. of the CDS breaks down the first-year admission stats and Section D. breaks down the transfer rates.

In general transferring into T20 schools is harder by a factor of 2 to 3 for top schools like Stanford, Harvard, Princeton, and Yale. About the same for Duke, Brown, UPenn, and UChicago. But actually easier for Cornell, Dartmouth, and Columbia.

Transferring to State schools like UCLA, UC Berkeley, UNC, and the University of Michigan is about 2 times easier than applying as a first-year.

Good luck.