Box hair dye vs Salon formula colour.....which is less damaging?

[nobodysfool]

I have failed.I have always been a bright cherry red colour but decided to give my hair a rest and go back natural (browny/auburn).
After a nighmare of a colour from my hairdresser (was meant to take it back to natural and ended up so dark like a Holloween wig). I colour stripped my hair and lightened it and bunged a light auburn ontop.I'm bored of the colour and it doesnt feel bold or bright enough for me.
So my question is what is less damaging for my hair long term?Box colour from the shops (which I have always used) or a salon colour which I can buy from a local hairdressers wholsale?
Any advice would be great. Thanks.

The 'bad' ingredient in hair dye is PPD (phenyl-something-or-other). No sure if it is damaging but it is the one with a health question mark over it. Hennaplus is a permanent colour with no PPD, which I use, otherwise it is really hard to source PPD-free permanent colour.
In terms of damage, I think the culprit is peroxide and the more in a formula and the longer it is left on the hair (i.e. for lightest blondes) then the more damage you can expect.
If you have long hair then the ends will get progressively more screwed with repeated colouring.
I use Hennaplus fortnightly to dye my white roots the same colour as my dark brown hair. For home dyeing, follow the time guidelines precisely.
I do wonder why Kate Middleton dyes her lustrous hair cos I fear it will end up straw-like, and I don't think she needs to cover up whites yet. I speak as someone who greatly resents the fortnightly home dye job, just to look halfway normal.

Swiddle- thanks ill look into it. I dont have any grey yet but I have coloured my hair for the last 15 years and just wanted to be a little kinder to it . With the money and connections Kate has I'm sure it will never look in bad condition.