For a small or medium size business, a VPN (Virtual Private Network) is a reasonable solution. If that is not feasible, then a dedicated network link, with all systems inside the corporate firewall, is possible - which is the solution most larger enterprises utilize.
As rubberman mentioned, the VPN solution is common because it is a low cost solution and can be used over a standard Internet connection. However, to be able to answer your question as to which solution is best, that depends on the business requirements and budget.
There is another way! You could use a direct point to point link! This will depend on your terrain and if you have access to towers or not, but is possible and even very doable. I am running several point to multipoint links with hardware put out by Ubiquiti Networks. For your purpose, I would recomend the Rocket M series radios. They are rated at 50km plus and one company has set up a link using Rockets at just over 100 miles, of course this was from one mountaintop to another, but still. If you set up on top of your buildings if they are highrises or put up rohn style antenna towers at both ends, You may get away with just that alone. You may have to put a repeater in the middle at 70 km. The potential savings for your company could be alot as you would not have to have a dedicated internet connection at both ends. The link would act like a long ethernet cable and you would be able to feed both ends with all services including internet. Essentially you are combining both networks into one all run off one DHCP server. You can calculate your links here http://www.ubnt.com/download#app. Just click on airlink. it is all done through your browser. You can find several places to purchase Ubiquiti hardware, however my two favorite are http://www.streakwave.com/product-ubiquiti.asp and http://www.wlanparts.com/c=N8Cr2I9WZz3Wk8HV6a4c7N7r3/category/wlan.ubiquiti/ . As you can see, the equipment is inexpensive compared to any other direct network connection and is very easy to use. Good luck, and if you have any questions, let me know.
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