Visio Standard vs professional

I'm new to visio I downloaded the 2013 professional trial the other day and loaded in my stencil's from several manufactures and they all worked fine.  I have decided that visio will meet my needs of doing simple boiler room piping and low voltage electrical layouts.  My question arises on which version to buy?  I looked at the website comparison ut it is very vague to the differences.  It is just me so I don't need multiple users working on same document or hyperlink to website capability so I'm leaning towards the standard version.  What has me questioning is the fact on the comparison page the customizable shapes box was a half circle versus the pro model.  They don't have atrial of the standard model to see what the half circle short comings are.  If it is just there is not as many pre-loaded stencils then that is not a big deal to me as most of the symbols I use are custom from manufactures.  but if it imply's that I cannot import files then that is a big problem.   

Can someone please let me know the detailed differences between the two versions so I can make a educated decision. specifically as it has to do with the box Create professional diagrams quickly with an expanded set of built-in and customizable shapes  in the comparison page linked below.

Thanks,

http://office.microsoft.com/en-us/visio/microsoft-visio-2013-plans-and-pricing-compare-visio-options-FX103791591.aspx