Tablespace autoextend Total Size

Hello people,

I´ve oracle 10g with the autoextend mode On in the tablespace. My doubt is about the options that I can to choose for resizing the tablespace size.

The TOTAL SIZE is 200GB and there are 15GB Free, the growing medium is over 20GB per month. Which is the correct procedure?

Add more datafiles or resizing the tablespace?

More thanks.