Building a Large Local Services Directory - Subdomain Needed?

[SEOPA]

In 2012 we will be rolling out a directory of local services for our industry. This will ultimately be thousands of additional pages, with city/zip searching, individual provider pages etc.

The main reason is for UX -- providing local resources for our industry to compliment the online experience (we're an online B2C retailer)

My question is if there are pros/cons to putting this on a subdomain, or if having it on the root is ideal.

I don't see a huge influx of backlinks (making a sub fine) but I suppose that could change down the road. I do see some indexing benefits for new terms like 'service x in los angeles' etc, but that would also be fine on a sub.

It feels like it would be cleaner to keep separate and on a sub, but maybe we're missing something. We certainty don't want to hurt anything on our primary site which drives the business.

Thoughts?

[JamesNorquay]

To be honest you can run it off the sub folders on the website I have seen recently a big website which has a directory on it: http://marketing.com.au/

Also from a link juice point of view it would be better on the sub folder.

The issue is duplicant content from people copying other directories and placing the info direct on your site which can leave dup UGC.

If you website is established and already has ranks then I would go with the sub domain. you really need to put a internal case together and look at all options both positive and negative.