A buzzing servo usually means it's hunting for the center position. It's not a serious deal but it will wear out your servos quicker. I would look for binding in the control surface or linkage as that is usually what causes that issue.
On what servos to get for your other plane, that's really a matter of preference. Since you're dealing with a smaller plane, heavy duty servos are not needed. I would start by looking at the following:

• S148 an analog standard servo with bushed shafts
• S3004 an analog standard servo with bearing mounted shafts
• S3151 a digital standard servo with bushed shafts
• S3152 a slightly more powerful version of the 3151