mailto question

If you want it to fire up the visitor's mail client then the second one.

:D don't forget that there are clients who don't use Outlook (in 90% of cases this link will trigger Outlook)... what will happen next? outlook setup window will open and your client might loose any interest in your services or products (i've seen cases like these).

I would suggest to you address without a link, but if you still want one you might want to improve it like this:
<a href='mailto:[email protected]' title="Contact Us">[email protected]</a>

good luck