The 11-5000-62 is the standard Corotor II plus feedhorn for linear c and ku reception, and the 11-7128-62 is the wideband version with dielectric insert for linear and circular
c band reception and standard linear ku. The frequency range they receive is dependant upon the lnb installed and not the feedhorn itself.
I've used the 11-7128 feeds for some time now and totally love them. Your rig sounds like what I have set up for my 10ft DVB dish. Channel Master dish with a Proform H-H mount. If you will be able to see many of the atlantic c band satellites I would definately go with the wideband circular version. The performance is top notch on both circular and linear C. I've compared the signal level difference between the wideband version and the standard version and can see no 'noticable' difference in signal level. I'm sure there is some slight attenuation due to the dielectric slab, but in real world (hobyist) use it's never been an issue for me.
One BIG thing to be sure of when installing one the wideband version is to be SURE that you have the dielectric slab lined up at PERFECT horizontal level at the dish's peak due south.
Also I recommend spending a little extra on quality lnb's. Personally I like Norsat. For the serious hobbyist I usually recommend the 8115 C and the 4106 KU. These have a stability rating of +/- 100khz.
LNB at Norsat International