Of the Celestion speakers made in China my favourite is the G12H which seems to have a wider frequency response. I also like the G12M Greenback, but this has a very middley sound without as much top or bottom as a G12H. This can be good in a bright amp, but is probably not what you are looking for. I don't especially like the V30, which can sound harsh in the wrong amp (or even in the right amp!). The G12-75 is fairly bland and in my opion not worth considering.
Many amp manufactures use the V30 because of its higher power rating, (you only need 1 in a 50 W combo and 2 in a 100 W). As Celestion will give a good bulk discount if you use the V30 in your entire amp range you can save even more money.
Regarding the power ratings, if you are using two speakers of the same impedance, then the power output of your amp will be shared equally between you speakers. The power rating of the speaker determines how much power the speaker voice coil can take before failing; it does not determine the "loudness" of the speaker (see below). Most speaker manufactures are quite conservative about their speaker ratings (probably to avoid many failures in use). Additionally most amp manufactures tend to exagerate the power output of their amps (more watts = more sales). There are many "50W" amps using 2 G12Hs, so I wouldn't worry about the power handling of a G12H in your cab.
The apparent loudness of a speaker is determined by its efficiency: a more efficient speaker will produce more accoustic power per watt input. This is always quote in dBs, probably because if it was quoted in percemtage terms you would be shocked. Efficient guitar speakers are in the region of 5% efficient, ie 95% of the energy going into them is wasted as heat. Hi-speakers, where linearity it more important than efficiency are often an order of magnitude or more less efficient than this.
To make a speaker more effcient you can do a number of things including: increasing the magnetic field in the voice coil, reducing the voice coil gap, making the cone and voice coil lighter. As many of these manoueuvres conflict with trying to make a speaker that can handle high power, it is often the case that lower powered speakers have much better efficiency.
Hope this is helpful. If you want to know more about speakers then there is some really good info on Ted Webers site:
http://www.webervst.com/sptalk.htmlhttp://www.webervst.com/spterm.html