Let's get a couple of things into context.
1) Mata and Sneijder are both excellent players;
2) Mata is at his peak;
3) Sneijder is a year or two past his peak.
4) £35m is an AWFUL lot to spend on a 28 year old player who can only play in one position. And make no mistake, although Mata played out wide earlier in his career (in Spain), the evidence form his seasons with Chelsea and Man U is that he is ONLY a 10 in the Prem. A very good 10 - possibly the best, in fact. But he's no winger in this league. Especially not when we have Del, Mirallas and Lennon in the squad: all three are pacy wingers who fit Koeman's tactics. Mata is NOT pacy.
5) 7.5m euro (£5.75m) is an absolute steal for Sneijder. But he too is ONLY a 10 at this point in his career. The critical difference is that unlike Mata he has no prem experience. Can he make the transition?
6) There's more to a transfer than just the fee. What are the proposed wages for Mata and Sneijder?
7) If either of these two come in, Ross is moving back to CM. Long term this is probably his best position IMHO, but is this the right point in his career to move him?
8) If Ross DOESN'T move to CM now, who are we bringing in to play there? If we spend £35m on Mata, how does that affect our shopping for a CM?
9) We need a CB and a GK too...
I'd take Mata at £25m, maybe a bit more. But £35m is too much when you consider how many other holes we have to fill with the budget we have available.