My first response is 357 Mag. You can shoot 38 spls all day. You get more powerful loads by simply buying 357 mags... As suggested, ream it to maxi and you have the best of all worlds...
BUT, the 45 Colt has long been my favorite handgun caliber. Sometime around 1988 I bought a Marlin in 45 Long Colt. I am a reloader and I soon discovered this thing was Thor's hammer on whitetails. Today there are a number of companies offering loads that duplicated my loads that worked so well. So you can load "cowboy" loads as plinking and Buffalo Bore or Cor Bon or a number of others for higher performance loadings. Federal used to offer a darn good defensive loading that worked very well on whitetails. IIRC it was a 225 LSWHP bullet at better than 1000FPS from a 7.5" barrel. Winchester Silver tips are another good one, but not as good as the SWC profile. Even with standard velocity loads, and using a SWC profile bullet, its a very good close range deer round. Stay away from the old 255GR pointed bullet profile for a big game hunting bullet.
BOTH will only improve once you start to reload. (Oh yea, you KNOW that your gonna do it!!
) If you ream the Magnum to Maximum. That one REALLY gets great. The maxi is a great handi caliber!
The 45 in the 20" barrel, can be safely loaded to the velocity heels of the 454 Casull from a pistol. That one has effectively been used for virtually every animal walking the earth today...
For ease of acquiring cartridges the 357 Mag wins.
SO, I say either one would fit your requirements of a good caliber.
The 45 is a better looking firearm, but with its stock its not so "friendly" with the heavy loads. Also its not friendly with scopes. You could add a strap on/lace on pad. OR you could chop it and have a grind to fit put on.
The 357 magnum, comes with a plastic stock, but in Monte Carlo so its scope friendly. The plastic stocks are pretty good and the forend is much better.
Either set is easily sold in the classifieds, to fund a set you may like better.
Good luck,
CW