Aluminum-framed 1911's work, but will exhibit frame cracks sooner than a steel frame because of the yield characteristics of aluminum vs. steel. Titanium splits the difference in weight and does not have the yield issue, since it is more elastic than steel. If you plan on absolutely shooting the **** out of the gun, get a steel frame instead of aluminum. Otherwise aluminum is fine.
There are multiple "plastic" 1911's, including the Wilson double-stack, Para-Ordnance, STI, SV, and probably others. The STI/SV frame design is
the choice for IPSC/USPSA, so you can be assured that it is reliable. I don't know about the Para and Wilson, but the STI/SV frame design is actually modular such that the actual frame (and serial-number-bearing part) is the frame (steel), while the plastic grip itself is replacable. This removes all issues with putting up with high round count, stress, and wear.
If I were to go with a high-cap 1911 carry gun, it would be based on an STI/SV frame.
If you just want a lighter single stack 1911, you can get everything from an Officer (or Kimber Ultra Carry) size (3" slide and reduced mag capacity in the shorter grip) all the way up to a Government size with an aluminum frame. If you want to go titanium, you need to get a pistol built on a Caspian frame. S&W offers a Scandium-framed Commander-size, now, I believe.
FWIW, I had a titanium-framed Commander built up for the purpose of carrying, and shooting the heck out of.
[ link to LARGER image ]
Here's a representative STI/SV (short slide - just one configuration of many possible)
-z