April vs October

We love both months for travel due to significantly fewer competing tourist. Weather can be a benefit, but you get what you get (early/late spring and Indian summer or early appearance of winter). We stay away from water dominated destinations during both months (Dalmatian coast/Greece, etc), but your destinations we would venture to explore.

I've visited Prague early April and it was winter one day, spring the next. The gardens weren't planted yet, and the trees were just beginning to bud, so looked like winter. I did miss the grand gardens, so would recommend mid to late April. The mornings and evening were brisk, but pleasant.
Safe travels.

We had snow in London the first week of April and cold weather in Paris mid-April. The vineyards in Alsace had bare grape vines, desolate looking. But no crowds!

On average I think the actual temps in most places will be comparable between the two months. However, in most areas, April has a greater chance of rain than October. On the other hand April has a bit more daylight hours. So, all in all, it depends a lot on what you want or plan to do in these locations. If a lot of outdoor activities, I would probably lean toward October. The earliest I've ever been to Europe was early May and weather in both England and France was lovely, but I don't know about April. I've spent several times in Europe in late Sept through October and enjoyed that time of year a lot.

We have traveled several times in October visiting Prague, France and Germany and had beautiful fall leaves on the trees.

We have also traveled in April to Prague and the weather was more variable.

I think that I might start looking at secondary factors, such as airfare. Which time period can you get the best priced flights?
Is there much of a difference in the price of accommodations?

We've decided to go ahead and go in April. We found good airfares, and there are a few other reasons that April works better for us for this trip.

Thanks to everyone for taking the time to share their experiences. And for the valuable advice!

Oh, one last point, I'd recommend avoiding Easter, because of the crowds , altho we found the Easter markets in Prague to be charming.

And I have one more point too....I traveled to France last year in April and I'm going again this year in April (right after Easter!).

I'm glad you have chosen April but IF you have pollen allergies, the horse chestnut trees (particularly in the Tuileries Garden) are likely to be blooming in mid-April. Last year it affected me so much I did about 5 Covid tests because I was convinced the symptoms were coronavirus. Nope, an allergy pill took care of it. So, I'm loading up on my OTC allergy stuff and will enjoy the blooming trees and flowers!

I'm pretty cold tolerant and to me this is a simply gorgeous time to go!