I recently created a web app called Kazu which aims to help speakers of Japanese and English learn to read numbers in both languages.
I found a web page which said that 'for numbers over 1万, "1000" is read as いっせん.
So, to be consistent with 一千万{いっせんまん}, I used 一千億{いっせんおく} and 一千兆{いっせんちょう}.
I'm also using いっせん where 1000 appears in other parts of the number, so 1万1000 comes up as いちまん いっせん.
I've tried to clarify with 2 native speakers whether these are the best choice for the app, and both of them said "it depends on the person", but they also said せんおく sounds better than いっせんおく — except when there's a counter word like 円 or 個.
Now I'm really confused. If you had to choose one "best" pronunciation for 1000億 and 1000兆, as in my situation, what would they be? What would you say for numbers like 1万1000? Should I change my decision?