I had a convertible carseat from birth - SNS Meridian. It has been great and I love how it can be rearward facing until 12kg which is safest. Apparantly there are some scary videos on you tube about forward facing seats until the kidlet is 'big enough'.
I have not had a problem but TBH because we live in the country I didn't drive that much (had a bassinet - beema Q) on and it was when we went to the big smoke we would be in and out of the car all the time. Then a capsule would have been good when little because you had to try and time feeds then sleeps with appointments etc because I didn't want to wake him to leave. But you do have to be careful about the length of time they are in the capsule for their backs. What we ended up doing was getting wherever we were going earlier and would just put DS to sleep in the pram.
A friend purchased a SNS unity and she found that her bub got too heavy in it well before she outgrew it - about 5kg. Apparantly the bub never stayed asleep getting in and out anyway.
If there is a next time around we might hire a capsule, will see. Our meridian carseat is so much more padded and supportive than the traditional SNS I am not sure it is worth it for the few days it would be an issue.

I had an emergency c-section and for the first week or two I had difficulty getting DS in and out of the car to put in the convertible as our carseat is in the centre. After a while it settled down.

Good luck in making your decision. You could always hire one and buy a convertible as well and if you have 2 cars have a carseat in each. I think it is only $60 or so to hire so is relatively cheap and if you don't end up using it you haven't lost huge amounts of money. Believe me there are plenty of things you spend $$$ on or get given and don't use