I have read many posts about it not being a good idea to hike in April or even May in Switzerland. But what about just visiting Interlaken, breathing the fresh air, getting over jet lag, taking the trains, trams, etc. up to the Jungfrau, a day trip to Grimelwald (sp?)? That is my plan. I plan on dressing warmly but is this a mistake? Should I just extend my time in the Loire Valley?
Don't bother going to Gimmelwald at that time of year. There's nothing to do except watch the locals fertilize the pastures (and be prepared to get some of that muck on your shoes). You can take the train to the summit of the Jungfrau, but this is the foggiest time of the year. You risk paying all that money to see nothing but clouds. Although I really like this region, it just isn't worth the huge cost to visit during the shoulder season. Extend your time in Loire, or visit another Alpine region that's more active at this time of year.
Okay, you ask point-blank if it would be preferable to try to do the Jungfraujoch and Gimmelwald in April, compared to extending your time in the Loire Valley. As a general statement only, I agree with the other posters: that late April is a great time to be in the Loire Valley and not a great time to attempt to go to 11,500 feet in the Alps--because of the weather, something you can do nothing about.