1. CORS is RTK.  RTK stands for Real Time Kinematic.  The CORS signal is an RTK signal.
2. Accuracy - A single CORS station will easily give "RTK Accuracy" (sub 2") at a twenty mile plus radius.  The quality of the rover receiver will be the determining factor on this.  The accuracy of the CORS station will beat an on farm base hands down.  The CORS stations are monitored for absolute accuracy.  Some one will know if the antenna is moved.  The repeatability from a CORS station is hands down the winner here.
   
3. The networks that Iowa and Minnesota DOT are installing are VRS (Virtual Reference Station) systems.  You do not get the signal from just one station.  The system uses all the stations that are in your area to compute a correction for your location.  It computes a signal as if you had an RTK base in the field with you.  You should have sub 2" accuracy any where on the network.
4. Reliability (ie. who do you call with the system goes down).  I don't get all this talk about the system will be down all the time because it is a government system.  WAAS is a government system.  How much has it been down?  The Coast Guard Beacons were government sponsored.  How much were they down.  I have a 24/7 number for our DOT to call if the system is down.  I also have a backup network that I can use to get the signal.  These systems are being used by a lot of people and businesses.  They will not be down for long if they go down.  If you are on a VRS system (see 3 above) the system will continue to work if a local station goes down.
5. Cost - I am using SPRINT as my carrier.  I had to buy a phone ($150).  I bought the InTime Software ($3,000 - $4,000)(not sure of the current price).  I have a bare bones phone plan (no minutes) for $25/month and then pay $15/month for unlimited data.  That is $40/month on the phone x 12 = $480 /year.  So I had the one time cost of the phone and software which is roughly the same as the cost of a rover RTK  radio.  I have a yearly cost of $480 (per unit) which is less than the cost of Omnistar or SF2 and more accurate.  You may need a cell phone booster (I don't use one).  I think they run less than $300 (one time cost).

With a CORS station, you do not have to maintain your own station.  You do not have to move a base station.  You do not have to have repeaters.  You can have 2 tractors running 30 miles apart (can't do that with one on farm base). 

I guess I am considered one of the "CORS bandwagon guys".  I have no connections to any company selling anything.  I am just a farmer using CORS for guidance.  I have been using the signal for 3 years now.  We needed a closer station so this year a group of local farmers worked with the Alabama DOT to build a station in our area.  We bought the equipment.  The DOT installed it and added the station to their network.  We are working hard to get a state wide network in AL.

If you feel more comfortable owning your own station, then by all means buy one.  But know full well ahead just what you are getting into.  I know in some areas, there are dealer owned RTK networks that will sell you subscriptions.  We did not have dealer interested in installing a network here.  The line of sight radios gave very poor coverage distance here.  The CORS system by phone works much better "HERE". 

Some of the older CORS stations were installed strictly for survey use.  They do not transmit a real time signal. They are used for post processing the data and will not work for guidance.  It will not be terrible expensive to convert them for real time use.  If you have a station in your area that is not broadcasting a real time signal, you might want to approach the owner with an offer to help with the cost of adding real time broadcast.