Thought I'd just put a word in, as I've made this exact decision. However, I think its devalued by saying cheap and quick...So, here was my thinking.
1. To get to the same end game, that of a full PPL(A) (or EASA equivalent) it's more expensive if you go via the microlight route. That is definite (I have a spreadsheet!). The total number of hours is likely to be more, and the microlight training counts minimally towards it. As others have mentioned as well, this is the only option for putting stuff like IMC and night ratings on. If that's what you want, then there is no doubt that training straight away on light aircraft will be both quicker and cheaper.
2. My end game was flying. I didn't care much about being restricted to VMC, as I only want to go flying on nice days (at least, at the moment). Microlight training is definitely cheaper per hour than light aircraft, so that's good. As for time to learn, I have been strongly advised not to consider that 25 hours is less than 32 or 45 hours. I've been told (quite sensibly) that it will take however long it takes. It may be a bit shorter for microlights as some of the instrument training is less, but both microlight and light aircraft pilots have told me not to expect one to be significantly quicker than the other, to get to a competent level. However, there are fewer exams and ancillary costs, so yes, it will be cheaper.
And after I get my NPPL(M)? Well, per hour flying will be cheaper as long as I can afford to buy in to a syndicate, and at that point, if I want to go on, I can convert. Overall, it will cost more, but why not wait till then to decide. I'll have earned more money by then too!
So, cheaper and quicker, yes, as long as its to the point where you can fly by yourself and have some form of license, not to the same license.
Lastly, one other piece of advice I was given was to have another think at about the 10-20 hour point. At that stage, I should have learnt the basics, and perhaps have done my first solo. Those skills [not hours] are (I am told) transferable, whatever. If at that point, I want to move on to light aircraft, I will be starting with the experience of how to fly, and even if I have to do another 45 hours for license reasons, the overall number of hours (55-65) is probably not going to be any different.
Hope that helps.
IPZ