Hello,
It depends what type of beach holiday are you looking for. Langkawi is a large island with many types of beach resorts which caters to all budgets/preferences. Langkawi, being a much larger island has a larger town and lots of different types of tourist places to visit such as Crocodile farms and duty-free shopping malls. So you can still have a beach style holiday and still go into town to visit all the tourist spots. But beware, as Langkawi is very touristy spot and with that comes all the usual touristy traps and scams.
Since it would be your 25th anniversay, you might be interested in more of a "romantic" or at the very least a more intimate beach holiday. In that case, I would recommend Pangkor Laut and this is a beach resort famous for its more romantic/coupley environment because of its secluded and out of the way location. But the place it lovely and the resort prides itself of its great service. I would definitely recommend this place to anyone who are beach bums and on a romantic getaway with great natural sceneries.
In regards to food, Pangkor Laut is such a secluded place that there are no other restaurants available on the island so you have to eat at the hotel itself. Because of this, the prices on the menu can be quite pricey and expecially if you include alcoholic drinks (since Malaysia is a Muslim country, the tax on booze is ridiculous!). You should check the website for the Pangkor Laut resort as they sometimes offer some great packages which includes all your meals. Langkawi will offer you much more freedom in terms of food. You will always have the choice of dining in your hotel or you can always hop on a taxi and eat at a restaurant in town instead.
In reagards to the weather, June is a good time to visit. The monsoon season is way over by then though you can never tell with the climate change these days. Rain in the tropics tend to be really heavy but unlike English rain, they tend to stop after a couple of hours and the sun comes out so I wouldn't really worry about that too much.
I hope that helps with your queries and hope you have a fantastic anniversary!