Opinions aside, let's look at the options you gave:
1) SMB
This may work with desired results, but SMB is not a good option for WAN-based destinations because of security risks (exposing your destination server to the Internet, for example). Otherwise, it is a good option with proper security precautions (firewall rules, etc).
2) VPN
A VPN alone won't do the trick. You'd need to use some other technology on top of it, such as SMB, NFS, or any other method. Also, depending on connection quality interruptions could affect your application.
3) SCP
SCP is a file transfer protocol using SSH and is similar (but as comments pointed out, not the same as) SFTP. This is a very secure protocol and will work well for your needs, with the side effect of overhead since you'd be transferring files in batches rather than continuously (or running scp x times a minute, for example).
Between the 3, SMB is the easiest to just get working providing you do it properly and if you're familar with how to configure samba properly.
if this gets closed then ask in chat but you need rep to do that unfortunately. It's an intelligent question, you deserve rep. Make a comment then people can upvote that at least. In the meantime better to ask in another forum. Over here they'll take your very good intelligent question and say it is argumentative or not constructive or some incredibly dumb criticism as the reason for closing, but what it boils down to is they don't like somewhat opinion based questions on superuser. – barlop – 2013-12-02T17:20:57.093
1Which forum would you recommend? – ashishlal – 2013-12-02T17:22:36.977
arstechnica forum is often quite good – barlop – 2013-12-02T17:22:51.243
2@barlop - You don't get reputation from people voting on a comment. This question has a great deal to be desired, for instance, specific operating systems in use. – Ramhound – 2013-12-02T17:29:00.653
2But what your concerns are? All methods will let you successfully accomplish copying files. There should be something that makes you to prefer one or another. If there are no such concerns I personally would prefer scp since it requires less configuration. – VL-80 – 2013-12-02T18:00:02.980
Since the commentary on the "on hold" message says "If this question can be reworded to fit the rules in the help center, please edit the question or leave a comment." If this is really an "if", then perhaps those that put it on hold with that message, could reword it? – barlop – 2013-12-03T19:31:24.890
This is not a forum. Questions must be specific and answerable. Discussions belong on forums or in the [chat].
– Kevin Panko – 2013-12-03T19:59:48.0601@KevinPanko I didn't say otherwise, but since the message that appeared when it was put on hold says "If this question can be reworded to fit the rules in the help center, please edit the question or leave a comment" <-- see what that says. So either it can or it can't. If it can then perhaps you can reword it? If it can't then what is the point of it saying "If"? (and notice also from his rep (23 as of writing and was even less before, as he is new, that he can't get into chat) – barlop – 2013-12-03T20:06:14.667
Well, you already reworded it, and I voted to reopen the question. It has 2 out of the required 5 votes right now. But it would be even better if the question did not ask "How can I do X" and was "How can X be done?" The best questions are able to help not just one person, but many people with the same kind of problem. – Kevin Panko – 2013-12-03T20:12:34.430