Not sure what you mean about the free arm. What makes you bring it up as a consideration? Perhaps high-end sergers have a free arm; not sure, as I have not shopped the all-singing all-dancing models. No lower-end sergers I have seen are really free arm machines.
There are a lot of reviews of the Brother on Amazon:
http://www.amazon.com/Brother-1034D-...dp/B0000CBK1L/
Couldn't find any reviews of the Janome on Amazon, but here's a photo.
http://www.amazon.com/Janome-7933-Se...dp/B00F9K76TE/
I guess I would buy whichever offers the most stable tension. I saw in the negative Brother reviews that it could easily be thrown off on tension -- something to watch out for in lower-end sewing machines as well.
If my choice were between these two machines only, then I would probably opt for the Janome.
What exactly are you planning to use the serger for? I think they are most useful for making children's clothing. A lot of us have sergers that don't get used much because there are no young ones in the family.