Are Sigma speedlight flashes comparable to Nikons in terms of quality for price?

I was thinking of getting a Sigma EF610ST for my Nikon D70s. The Nikons are overpriced and the Sigmas seem of good quality too. I already have a Nikon SB600 but it's not that powerful. I'm going to be doing some outdoor portraits of 3 children and I would like another flash. I already have a reflector also. Any tips?

You seem to have figured it all out, spec wise.

But. The Nikon SB series flash units, SB600 and higher optimized for use with Nikon's Creative Lighting System (CLS). When you eventually replace your aging D70, that will be important.

Sigma can't include any proprietary electronics that Nikon includes in their dedicated flash units like the Auto FP High Speed Sync Mode the Nikon flash units have.

Sigma is already in trouble over their alleged patent infringement of Nikon's VR image stabilization system.

http://www.photographybay.com/2011/05/26/nikon-sues-sigma-over-lens-stabilization-patent/?awt_l=PKpcw&awt_m=1cTbFTtDnv62xu

Note: How far away from your subjects is your SB600 that you say is under powered? I use a SB600 at 50 feet for flash fill a few times a month

Sigma speedlights are good - I don't have one personally, but I know people who do & are very pleased. It has TTL (auto flash metering) capability, so will do fine as long as you get the Nikon TTL version.

If you are taking portraits, you really need to get that flash OFF camera, especially as you won't be able to use bounce flash outdoors (other than to bounce off your reflector). The SB600 is a perfectly good flash and with your D70s, can be used wirelessly off camera in TTL using Nikon's CLS system (your on-board flash can act as a commander).

If you are prepared to learn how to adjust a flash manually, there are cheaper options. Although I have several Nikon units (couple of SB600 and an SB800) I also use Yongnuo flashes - a YN460-II costs around £40 and a YN560 about £60. I use radio triggers (RF602's - about £25 a set) to fire them off camera, which are very reliable.

Suggest you take a look at the Strobist blog if off camera flash is new to you;
http://strobist.blogspot.com/2006/03/lighting-101.html

The Sigma flashes can do high speed sync, although you may have to get the Super version instead of the ST version to allow it. The ST doesn't have all the bells and whistles of the Super.

Other than that, the Sigma units have slightly less build quality, but work just fine.

I found it interesting that you stated the Sigma was much more powerful that the SB-600 it isn't, Sigma just cheats and gives it's power rating with the head zoomed out to 105mm, GN61m, at the 35mm setting the factory rating is GN36m and Nikon rates the SB 600 at GN30m for 35mm setting, those are actually pretty close, factor in better build quality, better CLS compatibility and better compatibility with future Nikons and it's a pretty close match, B&H no longer carries the SB-600 but I gave $200 for mine the Sigma sells for $165… Again a pretty close match in terms of everything but quality.