I suppose it depends on what you are looking for in a convertible laptop. There's the Sony VAIO Duo 11 that my fit your needs, but it only has a 11.6" screen.
Anyway, Dell's design is a bit better when using the laptop in tablet mode because the back of the display basically folds down onto the keyboard so that the keys are protected. The Lenovo Yoga's keyboard would be the under side of the tablet; while the keys are deactivated, they are still exposed.
The Dell is lighter primarily due to the smaller 12.5" screen vs. the Lenovo's 13.3" screen. Therefore, it should be less tiring to hold when used in tablet form. The Dell's resolution is great at 1920x1080 while the Lenovo is 1600x900. That's good and bad. 1920x1080 means the Dell's desktop is larger than the Lenovo so that you can see more on the screen at once. But the downside is that 1920x1080 resolution on a 12.5" screen might be difficult to read so it is best to check out a similar 12.5" laptop in person to determine if the Dell will be comfortable enough for you to use.
The Dell also has a slightly faster CPU, the Core i7-3517U vs. the Core i5-3317U. That's a 200MHz difference for the base clockspeed and up to a 400MHz difference with Turbo Boost. So the Dell can potentially last longer before you considered it to be too slow for your needs.
Overall, my suggestion based on just a preliminary review of the two laptops is that the Dell is the better choice. But only if your vision is good enough for 1920x1080 resolution on a 12.5" screen.