I can't decide between the 3Tb Seagate Constellation CS SATA 3.5 hard drive or either the 256Gb OCZ Vector SSD or 256Gb Samsung SSD 840 Pro as they are the same price but have such a big difference in space.
I currently use a 120GB SSD for my hard drive and current games but only have 10GB left and want to free up room on it. I also have a 1Tb SATA 3.5 which I use for my movies, photos, music and anything else I need.
It will be used as my gaming hard drive and possibly hold my movies, photos and music in the future (once I use up the 300GB left on my 1TB SATA 3.5)
What would be the read and write speed of the Seagate 3Tb hard drive?
Depends on how much storage you need. SSD will only decrease your load times in games and while SSD prices have fallen, they're still fairly expensive to run games from, since some games can easily eat up 20-30GB a piece and the benefits are somewhat negligible. Of course this depends on the amount of games you have. So, if you only have a few games and don't mind deinstalling them regularly to free up space, you should be okay with 256GB.
Mechanical drives are sufficient though for movies, music and pics. You're unlikely to notice much of a difference in seek/access times and these files use up tons of space. Which is why, for now, I'd still opt for a mechanical drive as storage, since your paying tenfold per GB on SSD. Just be cautious with wear and tear, backing up your files is always a good idea and since you already have 600-700GB, it would cost you a lot more to secure these on SSD.
As for your other question, I just bought a 2TB Barracuda, not sure how these compare to the Constellation, but mechanical drives shouldn't vary too much, thus you're probably looking at 1:5 write/read speeds and ~12.5ms to 0.1ms access times compared to the 940 Pro.
Personally I have a 1TB WD Green HDD for storage and then a 256 Samsung SSD pro for my OS and all other programs. The loading time difference is night and day. Anyway, if budget allows for it I would get an SSD for programs and an HDD for storage.
Also, under most benchmarks the 256 pro is currently the top market performer.