It is very difficult to do an apples to apples comparison. There is a wide variation based on source and brand of LED lights... not all LED's are created equal. There are however two key factors
1) SMD refers to the number of LED's on a bulb, the higher the number the brighter
2) color, there are 3 main colors available Bright White, White and Soft White. Bright white is more institutional, almost blue but much brighter, whereas soft white is more like incandescent, almost yellow but not as bright. Color is a preference thing.
I replaced all my 921's with 42SMD Soft white. They are a fair bit dimmer than the originals but I prefer the color. I did a review on my blog and did some brightness comparison
Review
