I just got the 3.8 compact a couple days ago and I love it. It's got the 11rd flush magazine for carry (I suggest adding the Pearce grip extension for the pinky), as well as the 16rd magazine for use at the range. It gives you a little more felxibilty as to what you can do with it than I think the 4.5 can. Granted, if you only plan on using it at the range, the 4.5 would work beautifully.
I have carried and do carry both the 4.5 and the 3.8. No difference really in comfort between them. Grip is same size, and the muzzle length really doesn't affect carry comfort between the two guns. It comes down to a good holster and belt, really.
Having owned both(I still have my 3.8), my recommendation is like everyone else. Just for range time or night stand duty at home, go with the 4.5. However, if it's going to be your CCW firearm, I'd go with the 3.8. When I had my 4.5 I used it for CCW, however had issues with the barrel length. I don't do IWB holsters with my auto's(only do it with my snubbie .357), so I have to worry about the barrel sticking out from under my shirt.
The 3.8 offered me what I was looking for. Just having it that much smaller in barrel length fixed the problem. Granted you can use a 4.5 in a shoulder holster rig(which I did during the Winter), however on your hip it's a bit harder to conceal.
Either way, you can't go wrong with either gun. Just depends on what you're wanting the specific gun to do.
I have a 4.5 that is a range/IDPA gun. Thinking about picking up the 3.8 compact for concealed carry and home defense. My thinking is that I train with the 4.5 for IDPA, so might as well make the most of that by carrying something similar. The 3.8 compact with the flush magazine can be my carry gun, with extended magazines for carry reload and home defense use. It can live in my bedside gun vualt with an extended magazine and a light on the rail, and when I carry the light comes off, the extended mag goes in the mag pouch, and the flush mag goes in the gun.
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