Electric artificial horizons tend to be considerably more expensive than electric turn co-ordinators or turn and slips. As stated they are easier to damage in unusual attitudes. If it is an either or situation, the slip information is probably more useful to a newer pilot for improving his piloting precision. It is certainly a lot more useful for spin avoidance, if you let yourself get that far...
In my opinion, if gyros are not installed, the pilot is much less likely to extend himself into IMC conditions. A compotent IMC pilot should have no problem flying decent stable flight on needle ball and airspeed. VSI and altimeter are a bonus.
Having delivered a helicopter not equipped with any gyros whatever, 1900 miles, it was easy to remind myself to not fly without reference to the ground even for a moment temptign as it might have been. I expect that generally, pilots of very light aircraft would be wise to also fly only with reference to the ground. If limited instrumentation reminds them of this, so much the better! Aircraft don't happen to get into IMC, pilots fail to plan and maintain flight so as to assure entry into IMC does not occur. It was a new and very worthwhile self disciplne exercise for me in the helicopter to remain effectively VMC at all times.