Sadly, when you trade firearms at a dealer, you lose quite a bit. He has to profit by the experience - and, usually, at your expense. An outright sale would be better - check local ranges for a bulletin board, if nothing else.
If your needs are for home defense, a 4" .38 Special +P is hard to beat. The .357 Magnum is great - for hunting - but can produce some serious collateral damage, as well as be disorienting, at the least, for the shooter when fired in a small and dark room. Consider 148gr full wadcutters - 158gr LSWC's - or the best, the +P-rated 158gr LSWCHP - as protective rounds in the home. To launch them, a venerable old Model 10 is ideal. better yet, especially for cleaning, is the SS version, the 64 - or the adjustable sighted 67.
For target shooting, or plinking, you cannot beat the frugal nature of .38 Special. All you gain by plinking with .357 Magnums is the need for better hearing protection, a bit better, abeit brief, illumination, and a bit more soreness in your hand and wrist. Of course, the hole in your wallet will be considerably greater, too. Price the ammo at your favorite store - or Ma-Pa ammo makers.
As to the GP100/SP101 - they both will need some at least semi-professional 'help' to improve their triggers, as they are delivered new as 'works in progress'. The SRH uses the same lockwork as the GP100, and quite a few sources for folks - or instructions - to tune them up exist. The SP101 is just a somewhat miniaturized version of the GP100 lockwork. Years ago, my SRH only needed some dry-fire to 'loosen up'... my LNIB SP101 this past January needed the 'whole treatment'... including springs - the directions can be found in the Ruger forums, if you are mechanically inclined.
Of course, my larger collection of S&W's simply needed dry-fire breakin to slick them up - and some Wolff springs helped, too. They are better, in my opinion, than the Rugers. Of course, either make will 'work' right out of the box... just not as slick as they could be. Remember, both brands want to insure that you can light off any ammo - thus the stronger hammer springs.
Stainz