According to google maps, it is 26 miles and 35 minutes longer via 281. Neither highway has a lot of hills and curves of any real significance. 281 goes thru a lot of smaller towns with traffic lights and reduced speed limits. But it does bypass Austin, which can be slow on I-35 sometimes. And of course, I-35W goes thru Ft Worth which can also be slow at times. I-35 is a very good highway with much of it newly reconstructed. BUT. There is still significant construction going on from Temple northward for about 25 miles. But even much of that is already on new pavement. They do a very good job of keeping the traffic moving thru there.
Both are good highways, and time and mileage difference is not significant. Just a matter of preference.