nadja,
i think it really depends on what form of practice you want to study. do you want a ttc from sivananda? it's important to follow the path that is best for you. if you are unsure, i highly suggest going uncommitted and taking classes at various centers and ashrams before you commit to any ttc. why have a certification you don't want to use? fine if you've got oddles of time to spare, but a month-long ttc that you decide is not your style is a bummer.

but, there's a nice sivananda center 7km north of uttalkashi, and osho's center in pune is supposed to be great. rishikesh has many classes you can take and some ttc's, such as trika, which is a combination of asana, krira, and tantra. their ttc is month-long for level one, and you can stay with them for one year (further certs). the main ashram in rishkesh (i forget the name~they host sunset puja on the ganga) has special programs~they have an annual yoga conference followed by a 13-week (i think) iyengar course. karen bannon, responsible for brining iyengar to the u.s., is part of the conference and teaches daily during the conference~for free. donations accepted. and mcleod ganj area (dharamkot) has many centers, i believe offering ttc at an iyengar center there, and elsewhere. there are many nice ashrams you can visit and take their courses of one week or more, which could give you a nice introduction to different styles. phoolchatti, for example, offers various retreats, and the regular course is hatha in the am and ashtanga in the pm. retreats and courses such as these will also include meditation, puja and kirtan, and ashram way of life. once you're in india, you can talk with people at centers and ashrams and find out where others have been and what they liked. things can change quickly, such as instructors and retreats, so it's good to find out up-to-date info..

how long will you have for your trip?

In winter 2008-09 Sivananda Centre in Kerala had a lot of bed bugs!
And they were not willing to do anything about it, just told the students to put your mattress out in the sun.LOL
Like that's going to help all the bites you get over night.