For world crowns I would go with PCGS. NGC has the larger market share at present, but that will likely change.
If you're based in the US I can understand the reasons for slabbing, otherwise I agree with Tom goodheart. Slabbing in Europe really hasn't been embraced with the enthusiasm of Asian and US markets.
I would further say, but this tends to be one of my rules, from which I waver sometimes - don't slab coins that have a market value of less than $500 and preferably ones that will grade above MS64.
As a long term investment, I've written elsewhere, consider that the returns on certain coins (NON US) are typically higher outside of the US and even higher for circulated coins.
Food for thought