I personally would not touch the Janome 300e - only because it is lacking a few very nice things that they included in the 350e - like an auto threader which to me is worth the price alone...
I like the Brother 770/780 with or without the Disney. It is a good machine as well especially since you can usually get one for about $850 or so.
I have owned the Janome 350e - sold it to buy the MB4 which is a 4 needle machine, really REALLY loud and heavier than sin. I hated that machine and sold it, bought a Brother 4500d and have not looked back since.
The biggest thing is - what do you plan to do with it? If you are planning on setting up shop, doing craft shows or commercial work than a home machine is not the way to go.
I personally would go for either the Janome 350e or the Brother 770/780. They are both good solid machines.