Look .. a lot of what you’re asking requires a good deal of presumption.
1) what kind of connectivity is available in both locations.
2) can the same service provider be used to service both locations?
— case in point, #2 … say you went with Cox or Comcast Business Cable, and took 20/10 speeds (20 meg down 10 meg up, if they offer it) … if *both* locations could get on Cox .. Or Comcast as the same carrier …. then a VPN between office locations would *more likely* be a good fit..
Why? Because your VPN from site to site would not need to traverse the ‘internet’ at large to get from one to the other .. it would *only* go the backbone of the provider …
3) VPN an option? it should be.. this would allow you to create multiple zones… 1 zone in each location is office systems .. and neither of the offices from one side shall see the other side’s internal lan … 1 zone will be your atypical DMZ with web servers or what have you .. and then the 3rd zone … at least at your A office side .. would be where your test computer things.. are located.. so that the other company can have access to them …
4) check to see if MetroE services are available (Metro “Ethernet”) Again you would VPN each other .. and these can be sub 1000 per side depending on the speeds chosen ..
In any case .. the main things to try to do, are to stop sending traffic across the lines.. that should go out locally … example in the extreme.. say in your A side’s office building there is a Sandwich shop … they run their own web server and take orders … why should your traffic to the sub shop go from your office …. out to the other office *then* go to the internet to route back to ISP-X into the subshop downstairs from you!?
Now.. the reason to skip the T1 .. depending on the number of users you are servicing … is because as the technology used in the web advances … more and more often you will find people “abusing” the bandwidth without consciously thinking they are doing so..
Add to this decade of Youtube and Facebook consumptionism … the data that you need to go back and forth from office to office, and you’re now putting-on Data Fights with datahogs going at each other in a battle royale.
So … unless security and absolute uptime with guaranteed government mandated SLA’s are a requirement …. high speed data connections in each location, with good VPN’s are a good way to go (..ie FCC mandates 4 hour SLA for Technician working on a T1 line … where your cable or DSL internet provider would service you “as best as possible”, with the only SLA being the amount they would pay you if their service failed)
Anyway.. lots to think about out … lots to look at pricing for … and you may want to look around for Data Management companies that can help you by doing actual resource analysis … and they they would help you by ordering the lines, etc..
Had good dealings with some companies in the past that were just absolutely awesome when it came to pricing, ordering, and turn-up … though it was in the T1 hay-days ….