Is there much difference in weight and comfort riding between an Aliminium frame with carbon seat Post and carbon front forks,compared to a full Carbon frame
In theory the carbon frame should be better at absorbing vibration and it should be lighter. However, you'll probably find that a high end Aluminium frame fitted with quality components is both lighter and more comfortable than a low end carbon frame fitted with low end parts.
Just because it's made from Carbon doesn't mean it's light.
Budget carbon bikes often have significantly lower end parts compared to similar priced aluminium bikes because the frames cost a lot more to produce.
would the Carbon frame bike be a faster machine?
A high end carbon bike should be both lighter and stiffer than an aluminium bike therefore yes.
However, at the budget end of the market you'll probably find that the aluminium bikes are both lighter and better spec'd which means the carbon machines aren't as quick.
The speed of the bike depends on a lot more than just what material the bikes made of. Unless you're going uphill to a certain extent the weight doesn't really matter either.
For example, time triallists fit heavy disc wheels instead of light-weight spoked wheels in order to go quicker.
