Which one is more useful?
Whichever one you can see yourself enjoying the most. It’s not the major that counts, it’s what you do with it.
In general, you can only do with a major what some employer will allow you to do with it. If your bachelor’s degree is likely to be your only degree, then you need to choose your major very carefully.
Most employers have some need for people who are expert at using computers, while very few have any specific need at all for people who majored in biology, bachelor’s level only. Your campus career placement office is a good resource for finding out early, before it’s too late, if that biology degree is going to be of much help in obtaining a good job.
The job boards usually have thousands of postings in large metro areas for computer science, IT and similar jobs, but only a few dozen to a few hundred postings at most for which a Bachelor’s in Biology will be acceptable for a biology-related job.
The salary ranges for the computer science jobs are much higher than those for biology jobs. The underemployment rate for biology is horrible, while the underemployment rate for the computer sciences grads is much lower.
So my vote goes to computer science over biology, assuming that you would work hard at either of them.
As somebody with an advanced degree in a biological field, well, everything that @DTBTSE wrote.
They’re both useful, but you should choose your major based on: 1) which one you’re more passionate about (so you’re more likely to work hard); and 2) which one you’re really good at (so you’re more naturally talented, at least compared to an average person).
If your interest and strength in each field is similar, then computer science usually has better major-specific job prospects than biology (however, there are occasional industry crashes, like in the early 2000s, where getting a computing job was extremely difficult).