The two sized systems we had were some Rocket Bottoms Totally Squared (RBTS) size Small made by my Gramma and the Swaddlebees MiniNappi. We received the TSs a few days after Squirt was born and they fit well from the beginning. The rise was low enough it didn't bother his umbilical stump even though there wasn't an umbilical snap like his newborn diapers. The MiniNappis had a higher rise that rubbed his stump, therefore we didn't use them very much until it fell off. Both had a narrow crotch similar to the Newborn diapers. This did make it difficult to stuff the Mini Nappi (a pocket diaper), but it wasn't a problem for the RBTS as it had a snap-in insert.
I didn't get around to trying out our One Size diapers until week two. We have a BumGenius Freetime and a Happy Heineys Pocket Diaper. The Happy Heineys has cross over snaps which allowed for a fairly snug fit. The BumGenius had velcro which also crossed over, but not as far. Both gaped slightly at the leg when Squirt was sitting in his bouncer which contributed to some leaks, but his legs chunked up to the point they no longer leaked by 3 weeks. The One Size diapers sported a wide crotch compared to the two size diapers. Since the Happy Heineys was a pocket diaper I was able to stuff it with a narrower insert which helped create a trimmer diaper. They both also had a higher rise which covered the umbilical stump. I preferred to wait until his stump fell off to start using the One Size diapers.
Here's a great illustration of all the diapers side by side. As you can see even at three weeks the BumGenius looks fairly bulky compared to the newborn diaper. The Happy Heineys is a little more bulky than the two size diapers, but not by much. Neither of the One Size diapers leaked past three weeks.
In summary, I preferred the two size systems over the one size and the newborn diapers. They fit well from the beginning without being much bulkier than a newborn sized diaper. Squirt is now two months old, 13lbs 10oz and he is on the largest rise setting on both the Mini Nappi and the RBTS small. However, he still has plenty of room to grow at the waist. At this point they have lasted twice as long as the newborn diapers and I expect them to last another month.
Even though the two size system was my preference, the right one size diaper is a realistic choice from a couple weeks old to potty training for a large baby. If I had to pick one of our two, I would choose the Happy Heineys OS pocket diaper. It was fairly trim when stuffed with a smaller insert and the cross over snaps gave a secure fit without wing droop. Both the bumGenius Freetime and the Happy Heineys also fit my 2 year old 30lb son. I usually keep one of the one size diapers in our diaper bag for outings since I know they will fit either one of my sons.