I put together a list of
current sidemount systems, that you might find useful.
In addition to dual-purpose (back/sidemount) BCDs, there are also systems that convert pure backmount wings for sidemount:
Sidemount Adapters. Useful if you are on a tight budget, already own a backmount rig and just taking the first investigatory steps into sidemount diving.
Dual-purpose rigs, being a compromise, will rarely offer the same performance as a dedicated, designed-for, sidemount system. As others have said, they can lead to frustration and diminished enjoyment. However, the benefit is that they can save you a lot of money, compared to buying two separate rigs for backmount and sidemount.
The first step in selecting a rig is to determine what is most suitable for the diving in your area. It helps to understand that there are two primary strands of sidemount design; essentially for cold-water (steel cylinder) diving and warm-water (aluminum cylinder) diving. More information about that here:
The Two Schools of Sidemount Heritage.
Once you've narrowed down that option, it is time to consider brand-specific factors; like wing buoyancy, need for redundancy, cost and bulk/size.
Buying a vastly over-sized wing can be counter-productive unless you have a need for substantial buoyancy. Bulk/size is important if you travel regularly. Redundancy (bladder) is beneficial if you tech dive (and needed if you dive/teach for TecRec without a drysuit).
Cost factor is an individual issue. Jay already posted a link for the UTD system at $315... that's a really sweet price. You could get that, plus a backmount wing/BCD for the cost of some complete dual-purpose rigs on the market.
