We could be more helpful, in general, if we knew your location.

If you like spicy go with Long Black Spanish. Holy cow those are hot. They can be grown in spring but are better in the fall. Funnily enough, if you pickle them their heat disappears.

The difference in descriptions for French Breakfast probably depends on how and when it was grown. Any radish grown at the wrong time or if not grown properly can be pithy and hot, but grown right they are crispy and mild.

Rodney

Concur: Black Spanish whether the long type or the round type are hot. Most of the Asian winter radishes are mild. Some of these can be grown the spring, but are best for fall. Zone 8 or higher they will stand through the winter. In colder zones they can be stored like turnips. Both salad radishes and winter radishes are susceptible to cabbage root maggots. Other than that they are one of easiest veggies to grow. For a salad radish that is hot, medium size, longer standing, I would recommend icicle Most productive and tasty for me has been Pink Beauty. For a winter radish I like the Chinese Red Heart/Watermelon, Korean, Tae Baek, Japanese (daikon) Minowase.