At first, please check this FAQ. FAQ has comparative table for this question.
td-agent has own Ruby environment so it doesn't affect your system Ruby environment.
In addition, Fluentd is a gem. If you use Fluentd gem directly, you should write init script or
manage Fluentd with another tools, e.g. supervisord or something.
We recommend td-agent if you have no reasons. It is easy way to use Fluentd.
I'm not sure. It depends on ABI compatibility between precise and trusty.
We, Treasure Data, don't have a td-agent release plan for Ubuntu 14.04.
But we will release td-agent 2 for Ubuntu 14.04 soon.
Masahiro