I have a SB700, a Yongnuo 565EX and use of a SB910. The SB910 will give you every possible advanced flash feature your camera supports. The SB700 is about 1 stop less powerful and has MOST of the advanced features. It does not for example, offer a C group when acting as a COMMANDER for off camera wireless flash. You may never need this and there are many ways to work around this. The Yongnuo 565EX is about 1/2 the price of the SB700 and offers TTL and all the SLAVE functions of the SB910/SB700. It is also as powerful as the SB910. It will not however, act as a COMMANDER for off camera wireless TTL operations. Some Nikon cameras, the D7000 for example, have the COMMANDER function built into the flip flash. The D5100 does not and in order to do off camera flash with the D5100 and the YONGNUO yn-565ex, you would need a SU800 commander in addition to the yn-565ex. The build quality of the YONGNUO is not consistent. The 1st one I bought works perfectly while the 2nd one, which I just received, doesn't sync consistently with the camera and will be going back. My over whelming recommendation would be for the SB700. It will work perfectly with your camera, is powerful enough, and it balances nicely with the D5100, the SB910 and yn565ex seem over sized for the D5100 (imho)