The issue of safety vs decocker is a practical one, based on two considerations: if you don't feel comfortable manually decocking the gun for the first DA shot, obviously get the decocker. If you're ok with the manual decock, there's no advantage for that purpose to the tactical.
There is also the grip issue - the two safety mechanisms (safety vs decocker) are in slightly different places on the frame, resulting in slightly different grip styles (I mean the way your hand and fingers wrap around the gun, not the physical frame/grip). With the decocker, your strong thumb needs to sit lower; if you're used to an 1911 style 'high thumb', you'll be decocking the gun during shooting, haha, so you'd want to get the safety. This is an issue which you can only decide handling the guns yourself - every hand is different, you need to check this for yourself. I had students who preferred one over the other, others the other way.
With a decocker, you have a lighter DA pull (since it starts from half ####, not all the way down) - if you're really worried about DA, this might be a factor for you to consider. Some people are really not comfortable/good/etc with DA, again, this is something to be decided on the individual level.
Re shadow vs regular SP01 - well, if you'll want to put adjustable sights on it anyway (I have these, work great:
http://ghostholster.com/catalog/product_info.php?cPath=28_67&products_id=299 (but I took out the FO from the rear )), the sights which come with it aren't an advantage, so we need to look at other things. The fact that there is no firing pin safety means that (a) the whole trigger assemble is much easier to work with, and (b) it's much easier to take apart the firing pin assembly for cleaning/etc. The safeties are wider, for more comfy thumb rest. I think someone also said an extended mag release, don't know much about that.
So, my advice: the choice is between an SP01 tactical and the Shadow - there's no point in getting the regular SP01 when the Shadow is available. The the choice between those two will focus on how you feel about the location and the feel of the frame with a decocker vs one with a safety, and whether or not you need a decocker (either because of the lighter DA, or because you don't want to manually decock). That's someting only you can know...