Before I can answer your question we do need to talk a bit about your systems HD SATA port.
In this series Apple offered a SATA II (3.0 Gb/s) SATA port so what ever drive you get needs to be able to run at this speed. There are two kinds of drives 'Fixed' & 'Auto'. A fixed drive is designed to work only on the speed it was designed for. So as an example a newer Fixed SATA III (6.0 Gb/s) drive will not work in your system as it wants to run faster than what your system can handle. Now if we have an 'Auto' sense drive it will work as even though it can run in a SATA III (6.0 Gb/s) system it also will shift down to the lower SATA II (3.0 Gb/s) speed as well! Some even go to SATA I (1.5 Gb/s).
So the next part how do you know what the drive is? Fixed or Auto. This can be tricky! You need to review the spec sheet of the given drive. So as an example here's a Seagate Fixed drive: FireCuda 2.5" SSHD and here is a Seagate Auto drive: FireCuda 3.5" SSHD. Note the 'SATA Transfer Rates Supported (Gb/s)' line. Even though these are the same series the 2.5" is a fixed drive! As it turns out the older 2.5" model was Auto sense (thanks Seagate for messing us up!)
Sadly, Western Digital didn't offer any auto drives only fixed but they did offer a SATA II models a few years ago so you'll need to do a bit of searching for them.
Both Seagate & Western Digital make good drives. There was a time point each had quality issues which got over blown. As they both had growing pains and natural disasters which impacted them, todays products are solid!
OK, last issue! Yes, you do have an issue with the systems cooling refer to this IFIXIT guide: iMac Intel 27" EMC 2390 Hard Drive Replacement jump down to Step10 you'll see there is a special cable connection for the system to access the drives onboard thermal sensor so SMC can manage the systems cooling. If you fail to install it your system will go into safe mode which is to ramp up the fans to max which is not good for the fan's and will make a lot of noise! Some of the drive makers don't all use the same pins and some have even dropped access you'll need to replace the sensor with an external one. As it turns out OWC has such a sensor: OWC In-line Digital Thermal Sensor for iMac 2009-2010 Hard Drive Upgrade which will allow you to put in any drive (HD or SSD) into the system!