There are several differences to consider when deciding whether to purchase the D5100 or D90. They both offer features that are different in a lot of ways. If you need don't need video auto-focus but do need an in-camera focus motor or CLS command mode, the D90 seems like a good buy.
First the differences:
*Estimated price you can get the d90 for
They both have:
http://nikonites.com/d5100/2498-d5100-vs-d5000.html
http://nikonites.com/d5100/2499-d5100-vs-d3100.html
http://nikonites.com/d3200/5672-nikon-d3200-vs-d5100.html
http://nikonites.com/d5100/2500-d5100-vs-d7000.html
http://nikonites.com/d5200/9382-d5200-vs-d5100.html
First the differences:
Feature | D5100 | D90 |
Pixels | 16.2mp | 12.3mp |
Sensor Size | 23.6 x 15.6mm | 23.6 x 15.8mm |
RAW | 14-bit | 12-bit |
Continuous shooting | 4fps | 4.5fps |
ISO | ISO 100 - 6400 / Hi-0.3 / Hi-0.7 / Hi-1 (ISO 12,800) / Hi-2 (ISO 25,600) | ISO 200 - 3200 / Lo-1 (ISO 100)/ Hi-1 (ISO 6400) |
Movie (highest setting) | HD 1,920x1,080 / 30 fps | HD 1280x720 / 24 fps |
Movie Auto-focus | Yes | No |
Monitor | 3.0 in. (flips to the side) | 3.0 in. |
Top control LCD | no | yes |
Focus motor | no | yes |
SDXC | yes | no |
Advanced Scene Modes | 16 | 5 |
Nikon CLS | Supported | Commander mode |
Battery | EN-EL14 Li-ion | EN-EL3e Lithium-ion |
Battery Life | 660 shots | 850 shots |
Weight | 19.7oz. | 22oz. |
Price | $547 (18-55mm kit) | *$799 - $1050 (18-105mm kit) |
*Estimated price you can get the d90 for
They both have:
- 11AF points
- 1 sd card slot
- Plastic bodies
- The same 3.0in monitor (D5100 swivels)


http://nikonites.com/d5100/2498-d5100-vs-d5000.html
http://nikonites.com/d5100/2499-d5100-vs-d3100.html
http://nikonites.com/d3200/5672-nikon-d3200-vs-d5100.html
http://nikonites.com/d5100/2500-d5100-vs-d7000.html
http://nikonites.com/d5200/9382-d5200-vs-d5100.html