I have been using Sygic Aura for about 6 months now and think it's great. I have never tried the Tom Tom app so I cannot make a comparison.
About all I can compare it to is a Garmin Nuvi which I have previously used in my vehicle.
The main reason I swapped to using an app on the iPad over the Nuvi was that I got fed up of having to set the Nuvi up all the time (for security reasons I didn't like to leave it permanantly set up) and that the dashboard mount was somewhat distracting.
The iPad can sit on my left knee area, or on the passenger seat, or my wife can use it for other things (and still hear voice instructions etc) and of course the iPad goes with me so there's no worries about a bit of technology sitting in the vehicle attracting thieves…
I find that the iPad/Sygic works anywhere in the vehicle and it can get a signal and work even in the floor well area – this is something the Nuvi couldn't do – it was VERY picky on the positioning in the vehicle.
I sold the Nuvi on Trademe in the end.
I find that Sygic does everything I want from a GPS app – I wasn't looking for anything bloated with features that I will never use – just something nice and easy to use that works reliably.
The map-base support seems really good – we had friends build in a new subdivision in Rolleston and I was most surprised that the first time we went to their house under construction Sygic knew the address and the new street was in there.
I've used it maybe 6 times per month for real-life journies and have yet to be disappointed with it :)
EDIT: Actually the only thing that is a little disappointing is the text-to-speech translation of some of our street names – in particular street names that are named after Maori placenames or things. But I guess that you cannot expect miracles in TTS. I have this turned off and have a rather pleasant British female voice simply give me directions without attempting street names…
When comparing the price of Sygic vs Tom Tom there's quite a difference!
I also recommend you so a search of "GPS" from the home page here at iPhonewzealand as the lads here have done some really good work on looking at what's available for New Zealand.
All the best with your decision :)