It's a toughie but I'd side with Tarte most of the time. The only Mally foundaton I really liked is the one in this
OTO and prefer it to the Tarte Full Coverage clay foundation. But now I prefer the Rainforest of the Sea foundation from Tarte.
The Mally shadow sticks are far better than the Tarte equivalent, which give poorer payoff and aren't bullet-proof.
I can't see me ever ordering the Tarte primer because at £27 for 15ml they're having a laugh. I quite like the Mally primer (in the pot) when I've had it in a set, but again probably wouldn't buy it specially. I've just bought the Rosie HW primer from M&S and it's lovely.
Both Mally & Tarte mascaras are good and I always have a stock pile from TSVs.
Tarte do nice powder eye shadows and they shrewdly recognise that there's a market for pretty looking palettes for "collectors" to..well..collect. I have a couple of Mally's early palettes and the quality is excellent; maybe there's been a conscious move away from Mally's powder shadows because the sticks sell so well?
Make up is such a subjective thing, my advice is to order stuff if you like the look of it but send it straight back if QVC has over-sold it and the products don't measure up!
What annoys me about Mally is that she's a completely different person when she's on QVC in the States; far calmer and informative. When I spoke to Leigh from Tarte she is indeed as orange in the flesh as she looks on screen - not a great look for a make up sales person.