I have been using Sygic for the last 1 year. I had done extensive research on the two before I decided to buy directly from Sygic. These were the differences at that time. I believe it is valid even now.
i) Sygic also uses Mapmyindia maps in India, so there is no difference in that respect if you buy MMI or Sygic. For most of the other countries it uses Tomtom maps.
ii) Sygic gives you lifetime map upgrades for free. After I purchased my product, I have got atleast 4-5 upgrades. MMI gives only 1 year map upgrades for free. I have seen Sygic upgrades are usually 1 quarter slower than when it happens in MMI website.
iii) The latest version of Sygic allows you use the same license in 5 different Android devices. This is a great feature since our household has 3 devices and I am getting navigation features in all. I don't think MMI allows you to do that. This was not available when I bought it but after it upgraded, it gave me that option.
iv) Sygic is an international product. So if ever you want to travel to another country, you can purchase the license for that country, download the maps and use it without any issue. MMI will work only in India.
v) Sygic customer support is awesome. They have a well established support forum, support.sygic.com where they listen to customer feedback and incorporates it into future versions. But since MMI does not own the navigation software, they cannot provide support on the product features. Downside is that Sygic cannot entertain any issues with the map data since it gets it from different providers.
vi) Supports TTS voices. This feature will help the navigation voice read the road names from the map. Though it does not support English (India) accent, so it is sometimes funny when it tries to pronounce Indian names.
vii) You can custom download the map of each state separately. So I have downloaded only the data of my home state and surrounding states where I travel. If you are planning to visit some where else, you can download the maps for that destination before your travel and remove that once you come back. This is a great way of saving your download time and memory in your phone.
Some negatives of Sygic based on my user experience:
i) I have seen many force closes and mobile freezes while using the product. Once it happens mobile automatically reboots. I have a Motorola Defy. This is very frequent when you try to go to main screen of the phone while using navigation or when a phone call comes while you are in navigation. The way out is that I use my Android Tab to navigate while I drive and keep my phone free for phone calls.
ii) As I mentioned before, map upgrades take up 3 months to 6 months to show up on sygic.
iii) Sygic does not allow map downloads to the computer and then transfer to mobile. So if there are map upgrades you need a Wifi to download to your device. It takes atleast 1-2 hours if you are upgrading 3-4 states at a time. Cannot imagine doing this in a 3G / 2G connection.
There have been complaints about Battery drain while using Sygic. What I have felt is that it is due to two reasons.
i) If you use the native GPS of the device, it consumes a lot of power. The way around is to buy a separate Bluetooth GPS Receiver and download either
Bluetooth GPS or
Bluetooth GPS Provider from Google Play.
ii) The display is on while the navigation is on, leading to battery drain. What I do while I go on road trips is to keep the device on only when I am nearing confusing junctions or need to bypass cities. You can also invest on a cheap android device and use it primarily for navigation.