I was accepted to both. I applied to both because I like liberal arts schools and am interested in engineering. I have heard that both are party schools and that Bucknell is known for being preppy. I am also under the impression that Bucknell has a slightly better reputation.
My academic interests are bioengineering and neuroscience. If anyone knows anything about how the two schools compare or any information that might be helpful for me when making my decision that would be great. Thank you in advance!
I also got accepted to Bucknell (applied for the Bioengineering major), although since I didn’t apply to Union, can’t give you a comparison.
All I can say is that if you’ve visited those places personally, it may be easier for you to decide where you want to end up going 
Union is a fine school, but on both the Princeton Review and US News and World Report Bucknell is ranked significantly higher.
Bucknell is also larger. Union is about 2200 and Bucknell about 3600. What major were you accepted into? As of a few years ago, biomedical engineering was impacted and didn’t have room for students not originally accepted into that specific major so, check for the status today but that may push you towards Union if you have your heart set on it. I know neuroscience is quite popular at Bucknell but I don’t know if there are any barriers to transferring in. Definitely check with both schools about access to your potential majors if you weren’t accepted into one of those programs. Congratulations and good luck!
A note on USNWR rankings and other measures: In overall score, Union’s separation from Bucknell (5 points) is less than the difference between the first and third place colleges (6 points). Union’s SAT scores are listed as slightly higher than Bucknell’s (though footnoted as SAT optional).
I think Bucknell has a much stronger reputation and quality of student in general