Which coast to consider for first time travel to CR?

Your location should be determined by the activities and atmosphere you prefer. The weather in your area of preference can then determine your travel dates. Be aware that during the Christmas and Easter Week holidays, the Costa Ricans head to the beach in droves. And the prices are higher in High Season, from December through April.
PACIFIC SIDE – central to northern region: Most developed beach resorts. High rise hotels with many amenities. Drier climate. Good surfing. Mostly Spanish spoken – not so much English spoken by the locals, except for the staff in the better hotels. Caters to Gringos – lots of American music. Tourists are mostly all from the U.S. and Canada.
PACIFIC SIDE – southern region (Osa Penninsula): Wild and primitive jungles. Lots of rain. Awesome untouched beauty.
CENTRAL VALLEY – Lovely rolling hills. Breathtaking views. Quaint mountain villages and farms. The people can be a bit shy, but friendly. Almost no English spoken. Extremely comfortable temperatures, day and night.
The CARIBBEAN - Enchanting uncrowded beaches. Amazing jungles. Mountain vistas. Less developed but with accommodations from primitive and cheap to five star. Very humid. Lots of rain but lots of sun, too – often in the same day. Good surfing at some times of the year, good diving and snorkeling at other times. English spoken everywhere due to the Afro Caribbean culture descended from Jamaica. Warm, helpful, friendly people. Lots of Europeans. Tourists from all over the world. Relaxed, laid back, inviting atmosphere. Lots of music – Reggae, Salsa, Calypso and American.
Culture: The only place you will find theater of any consequence is San Jose and it is not a small city. San Jose is where the majority of cultural events take place. I live on the Caribbean side in Puerto Viejo. Expats from all over the world live here. We often have cultural events, in fact this weekend is Arte Viva, an arts exposition with visual art, dance, medicinal plant workshops, yoga and more.

The Pacific side's rainy season is generally from April/May to Nov/Dec. The dry season is earlier further north.

The Pacific side has many more beaches than the Caribe side due to it just being about 3 times longer than the Caribe side, but the snorkeling is better on the Caribe side. There are many, many uncrowded beaches on the Pacific side and the highway is very good so you have fast access to that side from San José.

There are many Europeans on the Pacific side as well as folks from North America, and English-speaking folks are not hard to find anywhere in CR. Even if hotel staff doesn't speak English you will find them very accommodating and helpful.

Have a nice visit!

Casey  -  A Dull Roar - Moving to, Retiring in, and Living with Costa Rica