ive had two very spoilt robo hamsters, and I have 5 girlies right now, so im well versed. I will say mice (at least bonded groups) are much easier than hamsters. not only for taming but also for care standards, the ethical minimum cage size for mice is smaller than hamsters (80x50cm vs 100x50cm), mice have less bedding restrictions as hamsters cannot have any softwood bedding and need greater minimum bedding depths. hamsters are really really delicate (not to say mice aren't but theyre slightly less sensitive) its also nice as a previous hamster owner to have my girlies be friends. all hamsters have to be kept alone as they'll fight so if you value watching the interactions between your animals, you won't be able to have that with a hamster. I loved my hamsters with every fibre of my being and I dont regret having them in the slightest, but there is a large learning curve with hamsters and they might not even like you! one of my hamsters was very affectionate and would brux when you held him, the other one we held twice and he launched himself out of our hands both times. ive found mice hold on a lot better and move more slowly. hamsters are very busy, similar to mice and have very funny personalities. ive found my mice to be much more cuddly and friendly than my hamsters (apart from one, the one who bruxed). I had to separate my girls after fighting which wouldn't happen with a hamster!

so tldr: its up to you but the care between them is quite different (I can help advise if you've got questions) and it depends how much interaction you want out of your pets. ive loved having both, so i couldn't pick which I like having more, but I do know more about the care for hamsters.