The 1911 is really a wonderful rifle. Sure its long and heavy but its so well balanced it feels easier to hold than many rifles 3 pounds lighter. It will also give you some more accuracy over the standard K31s.
The 1911 Carbine is about the same length as the K31.
The internals are a tad different between a 1911 and k31. The bolt and reciever is significantly longer in the older model. This also means that on the 1911 the magazine sits a few inches forward of the trigger guard. It also means that the K31 barrel is a couple of inches longer than the 1911 carbine, even if the rifle is far longer.
The K31 has its lugs at the end and so can hold more powerful rounds than the older 1911, and handle about 500psi over both the older firearms.
Both Carbines are sighted up to 1500 metres at 100 metre intervals. The full sized 1911 is sighted up to 2000 metres.
The cost is the main difference nowadays. 30-40 years ago the 1911 was at most a $100 rifle but ammo was quite rare so those imported tend to have had little shooting through them. When the K31 first came out in the US it could demand $400-500 a piece though now with supply so high they are worth closer to $100 and if you can even find a 1911 the price has really gone up, and I seen them sell for $600-800 in places.
If you want a great shooter I would recommend the K31. If you can find a Swiss 1911 for any sort of reasonable price though its well worth picking up. Mine has become one of my favourites.