G Tech drive (6TB) and WD passport advice (4 TB)?

ON SITE

i am reconfiguring my computer and backup situation and am planning on getting a 6 TB G Tech drive to Time Machine backup a 4 TB "data" drive in my 2012 mac pro. this machine does not have thunderbolt so i am getting the USB drive. i guess i could get a firewire 400 drive but i am trying to keep the costs down. i believe the 6 TB is enough for a 4 TB Time Machine backup and i don't mind spending the money now since the 4 TB drive will have a lot of room for me moving forward over the years. that said this drive is 400 so if i can save some money going elsewhere i might be interested.

http://www.g-technology.com/products/g-drive-usb

OFF SITE

i was also going to try and get a western digital My Passport as an off site backup for this drive but i am seeing that this model does not have a 4 TB option. i seem to have good luck with the WD externals (knock on wood) and actually keep this drive in my truck in texas so i'm not excited about testing other models. does anyone know if this model in the 4 TB option is a good way to go? or, actually i guess i can't buy this drive since i don't have thunderbolt on the 2012 mac pro and it appears to only have a thunderbolt connection.

http://www.wdc.com/en/products/products.aspx?id=1240#Tab3

seems like the OWC drives also only go up to 2 TB

http://eshop.macsales.com/shop/firewire/on-the-go

THANKS in advance