Well, there are advantages and disadvantages with each method of carry. When you start to CCW you will need to modify your dress accordingly. Think also about how you spend most of your day.
For instance, if you are standing all day, a lower back IWB might be great. IF you're sitting any part of the day, this method will be uncomfortable.
If you drive for a living, consider a cross draw waistband holster due to ease of access and comfort.
As for the shoulder rig, wear a tshirt as the bottom layer (to keep the holster and the gun dry) then wear your holster, then wear a button down over it. drawing the gun will be slower than WB carry in SOME situations. If seated, shoulder drawing may be faster. if standing, WB drawing will be faster. for shoulder drawing you'd likely rip open your buttons and draw. for shoulder rigs, thinner straps will dig into your skin, but wider straps will be hotter. it's a preference..... consider counter balancing the weight of the gun with a magazine pouch on the opposite side, otherwise the gun may pull too much on one side from the imbalance.
i've carried a 5" Glock, the same as your XD, in a shoulder rig. it was heavy, but it worked until I got a 3" XD. I carry the XD IWB or outside WB in the Ross or Galco mentioned before. perfectly comfy.