Hi Dragoknit - welcome and beware!!! MJ bags are 100% amazing and will suck you right in!!!
I have the Blake and Stella. I love the Stella for days when I need to carry a lot of stuff around (I fit my 32 oz Nalgene bottle, work folders, mini iPod, charger, makeup bag, zip clutch wallet, sunglasses, cell phone, etc and it wasn't stuffed!). It's a gorgeous bag, but I don't find it very comfortable on the shoulder. I'm not a small person, so I know that it why...so if you are average weight or thin, you should be fine.
The Blake had to grow on me. I bought one off of someone I knew from another board, and I'm totally in love with it now. It's more structured than the Stella, but it fits over even my big arms and sits nicely on my shoulder. It's not so big that it is a pain to wear in the crook of your arm, so that is great too. I can fit all of the above, except the water bottle and folders (the bag isn't as tall to fit the folders), and have room to spare.
Both bags are pretty substantial - but I guess I'm used to it now! The weight only bothers me when I have other stuff to carry (grocery bags, shopping bags, etc) but it's worth it to me.
I don't have any info on the Venetia, but it's gorgeous!!!!
Good luck - let us know what you decide. And if you find a Stella (or any other bag) on eBay, be sure to post it in the "Authenticate This" thread...we have some AMAZING MJ experts on here who can give you great advice.
Dawn