Both are special place in terms of being considered as World Heritage sites. Your easy accessibility matters as per your place of accommodation. As most of tourist if you stay in Hotel, Guest house as general tourist then that could be Thamel (tourist area) and in different case as pilgrimage and cultural heritage package tourist would most of the time stay near boudnath or near swoyambhunath stupa. However, thamel is in the distance or 20 - 30 minutes walk from Thamel. Since your stay is short I could like to give you a quick knowledge that evening 5 to 7 in general route to boudhnath is mostly awful regarding traffic and time could be consumed an hour in road trip only and distance from Thamel to Boudnath is 6.5 - 7 km from Thamel. Now let's talk about timing... Morning is beautiful both places since it is pilgrimage site. Pilgrims pay homage to hevenly and feed birds and monkey in swoyambhunath and mostly these days morning time is covered with health conscious people too doing stretches and exercise there, that is common. Here Newari, Nepali and Tibetan community are seen present for pilgrimage and also one can see couple of tourists in the temple complex. Here you can not have easy access to the restaurant so if you are hungry then you have to run down the swoyambhunath hill. Rest room service is just two places in northern sides of the main stupa but one may have to walk down there. In boudnath most of the pilgrims are Tibetan and few Nepali and Newari. Here you can have easy access to breakfast in every blocks around more than 50 houses which makes the site a courtyard of circle. You will have good service of view of Boudnath stupa from any one of the restaurant from roof top to ground floor restaurant. Since hospitality services are in every steps here you have easy access to restrooms in any of those. I would recommend one to go to swoyambhunath stupa by evening if one wants to see sparking twilights in city which is benefit of being high place than the valley but This city has no such high end of special lighting in any where in valley which could compete western cities. The benefits of height makes me say is any time is good for swoyambhunath stupa and from hoapitality service point of view Boudnath is special any time a day. If one would be happy to know then I would say you can rent a room just around the boudnath stupa and enjoy its view but for in swoyambhunath in the temple complex there are no hotels or guest houses, one has to walk down to the bottom of the hill for such hospitality services. Now concluding here with information that both is World Heritage site so for preservation Nepali Government charges foreigners as entry fees and it could be around $10 per person westerners and Asian $40 approx. Last but not least, near boudnath, you can visit Chabahil where you can see Charumati Vihar, Ganeshthan and another small stupa there. Hospitals are found in this area. This is around 2km west from boudha and 5.5 km east from Thamel. Another site near here is Pashupatinath and Guheshowri temple both sacrosanct temple for hindu religion community. Around swoyambhunath is Kathmandu Durbar Square which is 20 mins foot steps away from swoyambunath stupa and 15 mins footsteps away from Thamel. And very last you can have all kinds of hospitality services, high end to low end gift shops, art training centers or s hooks for Thangkas and one and only Free training on Traditional Newari Wood Carving Place in the City, WOODCRAFT GALLERY where one can spend their time for learning and spending free time in the gallery space. I hope you must have got some relevant answer and if needed one can call me 9849501525, 982334339 for freelance Cultural and Traditional heritage Imformation plus Iconographic information.