Hi there.
I've used the Ergo pretty extensively (my husband and I own Magic Beans - thanks for the shout-out Allison!), and to me, the Patapum looks very similar in design. I would be surprised if it was significantly more comfortable than the Ergo. But now you've got me curious. :)
The Ergo is very comfortable to wear, and will certainly give you more complete weight distribution than a Slingling. After a certain point, a toddler gets heavy, and any carrier is going to cause some aches and pains. A wrap (like Moby Wrap) would give you the most support, but it takes some practice to master the backpack carry, and all that fabric can be inconvenient.
Definitely, if you're in the Boston area, stop by any of our stores. Our staff knows the Ergo inside and out and can help you try it on with your son and see if it will work for you.
And if you do get a Patapum, please let me know what you think - S.[at]mbeans.com.
You can also browse our online selection of carriers:
http://mbeans.com/baby-carriers.html