Hi Jun,
First of all, we at Nepal Trailblazer Trekking would like to wish you Merry Christmas and happy New Year 2015! And thank you very much for your inquiry about trekking tours in Nepal.
In response to your queries, I would say that the Everest Base Camp Trek and Annapurna Circuit trek can't be substitute to each other. However, these 2 treks offer very different views. On the Everest Trek, you start the trek from higher elevation (Lukla, 2804 meters above the sea level) and spend more days at higher elevation; and you will be surrounded by massive mountains during the trek. On the other hand, on the Annapurna Circuit trek, you start the trek at a lower elevation and trek through very varied landscapes, from rice terraces to a high pass of Thorong La (5416 meters) and reach dry-land of Mustang, similar to Tibetan plateau.
In a conclusion, I would say: To experience Sherpa culture and see the massif himalayas you should go to Everest Base camp and to experience varied of landscapes, meet different indigenous groups of Nepal, enjoy high mountains view, you should go to Annapurna Circuit Trek.
If you want to know difficulties of these 2 treks, I would say Everest Base camp trek is more challenging! If you are physically fit enough, I would say "Go for Everest Base Camp Trek".
In response to your another query, the mid March is perfect time period for both Everest Base camp and Annapurna Circuit trek. During the time we can expect fairly good weather with blue sky and good visibility of mountains. However, there might be flight delay / cancel between Kathmandu - Luka or vice versa even in March and April.
To check out details of the treks, price and availability, please visit our website (these treks are bookable online with instant booking confirmation by clicking on Book or Check out Dates and price availability tab within respective tour page).
Should you have any questions please feel free to contact us.
I look forward to hearing from you.
Have a nice and pleasant day ahead.
Best regards,
Tilak