I have tried both Spybot SD and Spyware Dr and I find one more user friendly than the other but is one more effective than the other? I know this could end up to be a matter of opinion, but preferences aside, I ultimately want the most effective one.

Any advice?

Each application finds its own share of infections... no single application can find everything. Most people use at least two.

Personally... I have 3, as well as using a few online scanners.

Each company has its own criteria about what constitutes spyware, so each will look for a slightly different set of problems. There are many anti-spyware applications, and having two or three will provide greater coverage. You will find many free AS listed as used by the BC community, so you can try each of these out and keep the ones that are easiest to use.
Regards,
John

Here is a useful list:

http://www.bleepingcomputer.com/forums/topic3616.html

A link for SuperAntiSpyware Free can be found in the list provided by jgweed which I have found to be very effective.

Spyware Doctor provides real-time protection but is not free unless you use Spyware Doctor Starter Edition.