Shillong in Meghalaya is just 100 kms from Guwahati city. Fine Roads, takes 2.5 hrs to 3 hrs max. Meghalaya is quite a beautiful place with multiple tourist spots. Easily you might be required to stay for 2 nights to 3 nights. Kaziranga is quite far around 200 kms approx and it takes 4 to 5 hrs max, so a day stay or a 2 might be advisable.
Guwahati would be the stop gap to go to Kaziranga. Bus service is regular every 1 hr. Costs 500 per person.
Guwahati requires 1 day and half to venture - the zoo, kamakhya temple, the cruise and the island temple
Kaziranga would require 2 days to explore 2 ranges completely. ethnic coocoon homestay is ideal to stay on a budget.
elephant safari is 1200/- and Jeep safari is 1100/-
Guwahati is well connected to Shillong by bus and cab. Cab is preferable with sharing 600/- per person. Shillong needs 3 days to go around. do not miss Laitlum canyon, Mawphlong sacred forest, wards lake.
Let me know your travel plan includes how many days, accordingly I can provide you an itinerary. Shillong and Kaziranga- both you have to travel via Guwahati, so staying in Guwahati will mostly depend on your preference, and won’t effect the travel plan much.