I use 3Tb drives im very familiar in Seagate drives (got all tools for it) and "might" get those really cheap so at time being have not changed brand.
What motherboard do you use? Look on your motherboard homepage for pdf manual and read that pdf manual im sure somewhere there is AHCI mentioned on sata ports. I did get noticeable speed from moving ide mode to ahci..
My tips would be somethin that plan your hardware first, what is your needs (Berkeley unix is sometimes very nifty on what hardware it accepts) then buy it when you get your hardware test memory and harddrives within 7 days or you might get shitty refurbished hdd's if you later noticed a bad drive, and if you not test hardware you will be later have lots of long nights trying to figure out why stuff dont work. Then you test your pool fill it 50% full what are speeds and smart data copy speed and smart logs to somewhere safe for "later usage" if you know what you gonna put pool what type compression would help pool and io. Dont use latest freenas software builds use what you know surely that works (i still use zfs version 5 pool 28 freenas 8 release). But then again if you test newer pool and software pool might be lot faster, or then not, thats why you need test those things first before you start using pool on normal day. Also i would keep my hdd's cool under 40c. These are my tips im sure someone would do an exact opposite, so in the end its you who does the testing and what is best for you.
Ah i meant dataset named syslog by systemfolder, i have mixed feelings on it so far have seeing slowdown when i created that syslog dataset.