The first is a safe open and close tag variation, the second is the so called short-open tag. The second one is not always available, use the first option if it's possible. You could check the availability of short open tags in php.ini, at the short_open_tag.
The primary issue with PHP's short tags is that PHP managed to choose a tag (<?) that was used by another syntax, XML.
With the option enabled, you weren't able to raw output the xml declaration without getting syntax errors:
<?xml version="1.0" encoding="UTF-8" ?>
Hope this helps.