I use both.
I started out with an HT and recommend that to any new ham. They are usually straightforward and hard to get into too much trouble with. When I first got my license I started with an HT. It didn't take long for me to start using it while out and about in the Southwestern Deserts. I got a hand mic, rigged up a mount and power source, and wala I hand a low power Mobil. A good mobile antenna was the next investment. This greatly improved the footprint of my radio and I ran with that for a few years. You can do a lot with a little bit of wattage and a good antenna setup. (Spend the time on your antenna setups!)
Then I finally got a good dual-band UHF/FHF radio. While the increase from 5w to 50w was cool and all, it was the convenience and additional features that I really liked about having a mobile radio. Things like a good external speaker, remote head, and even cross-band repeat are fun features to have. But operationally on the trail or helping out with public service events, I mostly use the Mobile the same as the HT when I had it in the vehicle.
Then I got my general license and ventured into the world of HF. My vehicle is my ham shack. I love getting up onto some high ground out away from the noise in town and reaching out to the world. My rig now is an all band all mode UHF/FHV/HF and that is my primary mobile radio. It lacks some of the features that the dual-band UHF/VHF have like the dual monitor and crossband repeating but I really don't need that anyhow and I still have my old one if I am going to need it. But unless I am doing Tail Com with a group I am usually only really using the HF side of the radio.
At the end of the day, I still carry my HT with me. If not just a backup in the bail-out bag, I also can rig it up for an APRS radio. I can use it as a loner for a spotter or new ham that is still getting started. And remember. in a legitimate emergency, anyone can use it for help.
Start with the HT and you won't regret it.