Personally, I like the Pioneer head units the best. More flexibility and user friendly. If your on a budget, then a new headunit might be the best option. That, alone, has shown to provide a better sound. The sound, your speakers make, are only as good as the sound that is sent to them. For cost cutting and to prevent damage to the speakers, auto manufacturers will skimp on sound quality of a radio. They will 20 speakers in a car, but they will charge you up the yang for it because people think "OOOOOH... it has 20 speakers!"
I worked in car audio from 1997 to 2013. I worked at some of the highest end shops and I worked at Best Buy. Every time a person told me they wanted amps and speakers for better sound, I always talked them down to a deck and speakers. Just make sure to run at least a 12awg constant wire from the battery to your deck. So you are aware, there is hardly any markup in a head unit. There might be a $20 markup on a $200 radio where speakers and amps have almost a 75% markup.
Here's a true story. A guy, with a H2 Hummer, came in and wanted extremely good sound. He was an audiophile. He purchased Arc Audio Robert Zeff Signature amps, Rainbow components, and a JL Audio sub. However, he refused to replace his head unit. After listening to the system, he became extremely irritated because he just spent $12k on a system and the sound was mediocre. The speakers sounded nothing like they did in the sound room. He was asking for a refund. I recommended a new radio. He refused and insisted it wouldn't make a difference. I grabed a $70 Pioneer radio, which was the cheapest Pioneer we had, and hooked it up to his system. His jaw dropped on how big of a difference it made. He ended up getting a $2k Pioneer in-dash touchscreen.