It does depend upon the cars you will be transferring it to - we have a Scenic and cant have the isofix one with a leg as there are hidden cubby storage areas in the footwell, which have plastic tops - so wouldnt withstand the impaxct of the leg and possible rotation in a crash. So we had to get a Britax older style one that had no leg.
The Isofix ones cost upwards of £200, so is there any merit in buying one decent standard fit car seat and then two cheaper (but high safety rating ones) for the other two cars, if for example, one car was to be used more than another, or two £120 ones for the main two cars and a cheaper £60 one for the (eg) grandparents car?
We have three cars that DS goes in and we have an Isofix one in DH - was a bargain, a MaxiCosi Tobi - not isofix but very easy to fit in my car, and my DD's older Priori in grandma's car. I fitted them all myself, so I know they are ok, and check them monthly. It suits our needs.
Having said that, we have an isofix group 3 seat for DD and I have to occasionally remove them from the car, and they are so easy to fit, and remove - so can recommend them!
You need to look at your circumstances, how much you can afford to pay out for seats (will be used for the next 2.5 years plus), and how much will each car be used.
I can recommend the Britax ones and the Maxi Cosi ones - historically they always come out tops in the safety stakes - M&P ones look nice, but are not so good.