I would say Tomo is very good if you have constant external exposure listening to Japanese, such as if you watch anime or dramas, or listen to Japanese songs.
The reason is because of their emphasis on speaking by "shadowing", which is listening to short conversations and then repeating it yourself. You will find that when you start watching animes/dramas, you will subconsciously do the same and link it to what you learn in class, which helps to reinforce your learning.
But as the poster above mentioned, if you are introverted and don't like to speak in class, then Tomo might not be the best for you. But I really must say that speaking actually helps you to remember much better than just pure mugging.
Ikoma is more textbook-mugger type but their JLPT prep courses are fairly popular. I took the N2 prep course and it was good preparation for the exam but you still have to put in effort to study if you want to pass N2.
i studied at Ikoma for 3 years (part time) and have been studying at Tomo for past 2 years.
It is like what FrostWurm said. Ikoma is practically read, write and listen focused as their ultimate aim is to make you work towards a JLPT certification (not mandatory but at least you get the hint that they are). If you need JLPT certification, I think Ikoma is a good choice.
Tomo has almost no focus on JLPT examinations. If you wanna take the JLPT, sensei will only give you pat on the back encouragement and depending on the teacher, you can reach out to them if you have questions on your exam prep. Otherwise, it is very much conversational based lessons.
Personally I took on Tomo's classes because when I went to Japan for the first time, I couldn't understand much when I spoke Japanese. After learning at Tomo, can say my conversational skills increased so much so that I can pretty much hold a short conversation with a native Japanese.
