In todays market would it be better to buy existing or build a new one

Buy existing. The market is a huge buyers market and you can get financing pretty cheap if you know where to go...and that does not mean traditional lenders. Go to http://www.thePowerofRealEstateNow.com to get instructions as well as financing. You can get a Line of Credit for the down payment and also 100% financing/100% LTV.
Joe Villeneuve
3venterprises.ws
joe@3venterprises.ws

It is less expensive to buy at the moment and you will have an easier time finding tenants as they prefer mature markets. If you build new you may sit vacant for a long time as most markets are over saturated as it is. The only exception is if you know of a very particular tenant like a laboratory, fast food restaurant, or drug store, who is willing to pay the rent you need to get your target return on investment.

That's a broad question that is market dependant.All states,counties,and cities have hot and cold areas right now and future land use maps. Intimate knowledge of the area you are investing in is cirtical to your success of a project or an acquisition.
If you can find the right land and build cheap in a hot area building might be better.Most of the developers and buyers I work with will look at many properties and make a choice and run there pro-forma and see if the seller and buyer can come together and make a deal happen.

It depends on your cash position...
The complete freeze of the construction lending and land acquisition lending market has had a crippling effect on land values and in some cases some very well positioned infill development sites. Finished product has held value more-so due in part to the availability of financing. Hence, the deepest discounts will be the land deals which - pending the availability of construction financing over next 1-2 years may be a better play than buying exising product. One thing is certain; cash is king.