There are no real cons for the beginning or high handicapper in either GI or SGI irons. The actual differences will likely be that blades or musclebacks will have a better look at address, little or no offset, a smaller "sweet spot", and more "feel". A mishit shot is immediately apparent with blades or musclebacks... so much so that your hands will sting as well as your ears. GI and SGI tend to somewhat disguise this mishit feel, but less so the resultant ball flight.
Usually, GI or SGI irons incorporate perimeter weighting that may lessen the effect of a swing that does not impact directly on the "sweetspot". They may also have a sole that has more "bounce" (a wider and more or less angled sole) to alter turf interaction, and a modified vertical center of gravity to promote higher launch angle, etc.
These MAY produce better results for those whose skills are not refined. The verdict is not really yet in as to whether this will aid or hinder one to develop a refined swing.
Edited by CR McDivot - 4/26/13 at 11:16pm