In reply to huckleberry:
I suggest trying both pairs on. See how they feel, how comfortable they are for you. The correct fit also has a big influence on insulation. Snug, no pinch points, ability to wiggle toes rather than being scrunched up etc. Personally I find heavier / thicker socks not very comfortable for summer alpine use. Remember to try the boots on with the socks you will be wearing.
Although both are rated the same stiffness (B2), the category is not consistent as there can be lighter less stiff B2s and heavier more stiff B2s. The less stiff are great for easier more walking based routes ( the ones you are alluding to ) but obviously provide a lot less support on steeper rock, snow or ice which you may venture onto. Often boots that are light mean not only less insulation but often much thinner layers, especially the vibram sole. This usually means they will wear out much more rapidly. So if you are thinking longer term or the ability to be comfortable on a wider range of routes the heavier ones may be the way to go.
Both boots claim -20, I'm not sure how accurate or realistic this is. Some people run 'warm' or 'hot' and don't seem to get particularly cold feet or hands. Some people run 'colder' and more quickly get cold feet or hands. Obviously this varies but generally, which are you? This may influence your decision.
Routes where you can move fairly constantly, such as the routes you are aiming for, help keep you warm but if you are stopping / belaying a more insulated pair may be better - what are your future goals as this may influence your decision.
Wjcdean hit the nail on the head when referring to higher and longer routes and also when you choose to climb. For example if spending a long part of a day above 4,000 metres ( such as the Trois Monts route ) you may prefer more insulated boots - but it depends very much on conditions. Early or late season can be very much colder. You can also get cold periods mid season. Sometimes you are walking on top of the snow, sometimes you may be plunging in. The more insulated boots may give you greater flexibility in terms of temperature / conditions range.
No one pair will do it all, you will have to make judgements according to your personal ability and the conditions on your chosen routes. On a 4,000 metre peak there are days and times for some people when really light running shoes can work, days when a lighter weight B2 are great and days when you are very grateful for more insulation.
Going lightweight ( whatever that may mean for you ) is great when you have the experience and expertise to make sound judgements when it is safe for you to do so. So, to answer your question, as you are currently an alpine novice I personally would recommend the more insulated B2 boots at this stage.
Bear in mind that many guiding companies would recommend B3 boots for something like Mont Blanc. I suspect that the 300 grammes difference in the boots you have highlighted will not be the deciding factor on safe summit success, having cold feet would be.
