I have both, as I installed Arnet DSL several months ago when the cable went out and after several days of unsuccessful attempts to connect to Fibertels customer service I succumbed to an offerta from Arnet for 6 months of cheap service.
Fibertel eventually fixed the problem, and I have continued with both since then while I decide which one to keep. My house is quite large, and wireless signals are hard to pass throught the thick stone walls, so I have been using Arnet on one side of the house and Fibertel at the other. I am just about to switch off the cable connection and keep the DSL, and here's why:
a) Speed of connection:
I forget what speeds the respective companies claim they are providing (Fibertel claims 2 gigs - hah!), and I have been in the Internet business long enough to know that the claimed speed seldom if ever has anything to do with reality. The bottom line is that here in Vicente Lopez, they are functionally identical, with the DSL perhaps a little more consistant.
Arnet is clearly cheaper, especially with the promos they run. Right now Arnet is charging about 50 peso/mo. (after discounts) while Fibertel is charging $125, netting about 80 after discounts for cable service.
c) Customer Service:
This is where Arnet is the clear winner!
I tried for several days to contact Fibertel, and it took literally days to get through busy lines and interminable waiting on hold (several times I gave up after waiting 1/2 hour or more). When I finally got through I then was given the common "we will have someone there between 12 and 5 PM next Thursday" bit. They did show up that day, and did fix the problem, several weeks after it went down! No credit for lost service, of course!
When I called Arnet they answered the first time, were helpful and friendly, and had a modem in my hands the next day! I had some difficulty getting a signal through the antique phone system that has been in this house since the 80's, and I finally called customer support for assistance. Again they answered promptly, were helpful, and although the tech and I figured out what was happening they still sent a tech the next day (!) to make sure I was up and running.
IMHO, at least here in Vicente Lopez, Arnet wins every catagory!
Hope this helps!