One thing not mentioned so far is grips. A new GP100 will come with Hogue rubber grips. They are not bad at all. However you can change them out at low cost for the Hogue "Tamer" version made for the Super RedHawk Alaskan. The GP100 and Super RedHawk take the same grips. The Tamers really soak up recoil making the GP100 a lot more pleasant to shoot with Magnum Loads.
Next the 3" GP100 is a Fixed Sight revolver, unless you get the Willy Clapp Edition, which has Novak sights dovetailed in, you have no sight adjustment. This might or might not be an issue for you.
The 4" GP100 with adjustable sights comes with a quick change front sight system. A very good thing. You can order a Gold Bead or Tritium Night front sight, and in a couple minutes change the front sight yourself. No trip to the gunsmith required.
The SP101 is a pretty fair (near K Frame size) 5 shooter. Normally the trigger is not as good as the GP100 trigger. It is lighter, this however means it recoils harder in the hand with heavy loads.
Since this is a trail and range gun, I would recommend the GP100. It will be a lot more shootable handgun than the SP101.
Get a Lobo Enhanced Pancake holster for it with Inner Hammer Shield, and it will carry just fine with a Good Heavy 1-1/2" belt.
http://lobogunleather.com/
The GP100 takes the same speedloaders as the S&W 586/686 revolvers. The Safariland ones are on sale at Midway till I believe the 28th, as are the Midway Pistol Cases. Considering you will probably not be speed loading it, the Safariland Comp II speedloaders should work fine for you. It is also fairly compact.
Safariland Comp II Speedloader: Product # 742807 $9.99
Midway Tactical Pistol Case: Product # 160870 $9.99
Hogue Tamer Grip: Product # 231159 $18.99
http://midwayusa.com/
Bob