I am looking at getting one sub boxed with an amp for my car.
Do the brand of an amplifier matter?
For example l can get this package http://trademe.co.nz/Trade-Me-Motors/Car-stereos/Amplifiers/auction-177484335.htm with a 1400 watt dominator v12 amplifier for the same price as the same package with a 350watt sony amplifier?
Which one would be better?
thanks

Yes.
Cheap amplifiers often exaggerate there power output chronically.
Always use the RMS rating when comparing amplifiers and speakers.This is not perfect all the time, but is far more reliable than any other rating.

Unlike most of there other stuff, Sony car hi fi isnt very good.

Its more of a question of quality and reliability.
The cheaper brands sells you the products capability, while the branded ones sells you the reliability and prestige of their products.

Plan and simple, you get what you pay for.

In the grand scheme of things, both yes & no. There is the long running theory of ''A watt is a watt''. Meaning, in theory, a $100 50x2 watt amplifier will sound the same as a $2000 50x2 watt amplifier. But there r different factors that can color or effect the sound. In essence, the best amplifiers should really only have crossovers & a gain knob. There r tube amplifiers that can effect the sound a little, there r full range class-D amplifiers that r not quite up to the clarity of class-A/B yet, but most people cannot honestly tell the difference. There r many new amplifiers that have DSP on board that can effect the sound also. Lastly, there is the argument of the quality of parts & materials used in the amplifier. Many companies make very good clean amplifiers, but a good number of them r running very high margins based off of brand name hype.