Vanguard FTSE Developed Europe vs Vanguard FTSE 250

They track different indices: Vanguard FTSE Developed Europe follows FTSE Developed Europe and Vanguard FTSE 250 follows FTSE 250. They also differ in cost (0.10% vs 0.10% TER), replication (physical vs physical) and domicile (Irlanda vs Irlanda).

Nobody can know that: they track different indices, so their returns will diverge, and past index performance is not a prediction. What you can control today is the cost, the tax structure and whether the index actually covers what you want exposure to.

Their fees are effectively identical (0.10% vs 0.10%), so cost should not decide this one. Look at index coverage, distribution policy and availability at your broker instead.

Both are distributing, so on this point there is no difference between them.

Yes. Unlike two funds tracking the identical index, these give you genuinely different exposure, so holding both is not automatically redundant — just be aware you may end up overlapping on the largest companies and doubling your rebalancing work.