They both allow a 3,000 random member mailing every 3 days, not all of them are going to open your message but you get the point, that is just the potential. Both services are free to join but I opted to go for the one-time offer, I also believe that is where the real value is at, and considering I've seen people pull 100 plus leads on a single mailing with these services, the cost to go gold rounds out your lead cost to pennies on the dollar. The one-time gold offer is for around 200 dollars each and if you stay on top of your mailings that's 115 plus mailings for the year-long membership, that's a lot of dang leads!
The reason I chose the top 2 list builders as apposed to any others is that their active memberships are more than several of the other services put together and the same people will not be seeing my offer over and over again. It's basically a wider audience with both paid and free memberships.
Once a member, there are hosts of other options within each service. Like uploading your banners, internal ads, link cloaking, credit building for larger and sooner mailings, if taken advantage of can really maximize your investment. As a paid strategy, you'll really want to use all that there is to offer within these services. I've not even begun to scratch surface of the value within a good list builder. In both services I use, it seems to open up more opportunity for the other ones and both have unique features to themselves. For example, I started out with ViralURL and had no clue what I was doing, basically just waiting around for my next scheduled mailing, it wasn't until I joined Listjoe that I figured out a lot of ViralURL goodies! You can open up offers from your fellow members and get credits, both applied differently in each service. Listjoe credits are built up for larger mailings and ViralURL credits can be used to mail sooner for added incentive!
As a beginning marketer getting your feet wet for the first time in a paid strategy, they are an early introduction of banner advertising, 2-3 line text ads, copy-writing, and some real basic pre-written HTML codes for you to use and make some easy affiliate cash off of as well. That is just an example of what's in store once you're inside but "List builders" are an awesome way to spend a little bit of cash and get some great targeted traffic and value for your marketing dollars.