Home A/C Repair/Replacement quotes are very different. Which one to go with?
I would call a third company.  They should actually try find the leak, not just sell you a new system.

What brand is your A/C system (Lennox, Carrier, Trane, etc)?  Find the local HVAC company that is an authorized dealer for that brand.  They will be most familiar with your system.

Once you find out where the leak is, you can make an informed decision from there.

Common techniques an HVAC company would use to check for a leak:


Spray soapy water on the fittings and look for bubbles.
Use an Electronic Nose to "sniff" around all the fittings (it can smell the refrigerant).
Add a UV dye to the coolant, let the system run for a week or 2, then go over the system with a UV light (if you see the dye, there's your leak).


(Expect to pay for the leak testing.  It can take a good hour or two to go over the whole system, and time is money.)

Once the leak is found, the HVAC company should be able to give you an estimate for the repair.  If the leak is something simple like a fitting on a coolant line, the repair can be cheap/easy.

If the leak is in the coils or the compressor, expect the bill to be steep ($thousands).  At that point, I would think about replacing the system.

Reasons to consider replacing:


You fix this leak, but what else is going to go in the system?  If the system has been running for a while without enough coolant, there is increased wear and tear on the compressor.
R-22 systems will be getting progressively more and more expensive to service.
Newer systems are way more efficient, so expect a lower electric bill.  You should be able to calculate the "pay back" period of the new system (how many years of reduced electric bills will it take to offset the cost of the new unit).
Check for Tax Credits and Rebates.  The Energy Star Tax Credit expired at the end of 2011.  However, you might be able to get a tax credit on your state or local taxes.  Also check with your power company.  My power company had a rebate of ~$300 for a high-efficiency AC.
Newer features on the new system (variable speed fan, quieter scroll compressor, touch screen programmable thermostat, etc).