The consecutive day global pass usually isn’t the best deal financially unless you’re a real whirlwind traveler.  If you’re in Europe for 90 days, you’ll probably be moving from city to city an average of every 4 days or 5 days.  That’s 18 to 22 days of train travel — and some of those trips will probably be short enough that a point to point ticket is cheaper than using a day of a flexi pass.  For some of those trips a cheap flight will save time and money over taking the train.

The consecutive day pass has the advantage of providing SOME (but probably not most) of your local transportation  and will also cover day trips and other short hops where the cost of the train ticket is less than the value of a day of the railpass.  You wouldn’t waste a day of the flexi pass for those trips, but you could use your 3 month pass. Those savings probably won’t add up to the difference between a 15 day flexi pass and the 3 month pass, but it will bring the cost a little closer together.

A Eurail pass doesn’t cover UK and much of Eastern Europe, so you’ll definitely have travel expenses, in addition to reservations, that aren’t covered by the railpass.  For a 90 day trip, you may be spending 1/3 of the time in countries where you can’t use your railpass — I“d be spending at least 1/2 of it in Eastern Europe.