TBH I didn't look at a free VPN. I've read in other places about unreliable connections (slow, dropouts) etc, so I looked for positive reviews and Strong were mentioned a few times.
Netflix recommend a min. speed of 500kbps, while Hulu recommends 1mb connection.
Thing you have to remember though is that because you are logging into a VPN, which is passing the connection to you, it will be more dependent on THAT speed than your normal internet connection.
Bad connection to VPN = bad playback.
I'm on Cable and I found Hulu pauses and stutters during commercial playback, but the shows themselves for the most part were stutter free.
Had no problem at all with Netflix.
Hulu don't state file size, but netflix says each hour of SD is about 1gb, HD is 2gb & you can set netflix to only use SD if you want.
I only have a 50gb data allowance, so this may become a concern down that track but we'll deal with that if and when it arises.