15L is just one medium sized bucket.
I have a 15L haier in one of my bathrooms... bought it only because of its timer function. At that time, it was the only company with such a timer. It readies the hot water at a specific time set, unlike other models which start at a set time. It is small for regular use but it has a selectable power option and therefore, it can provide next batch of hot water in 5~7 mins.
I have 2 more haier 35L models and they take around 11~14 mins for each cycle. I must mention another feature which I found unique to this series: it has two timer options - absolute and relative. And after reaching the set temperature, it can be set to maintain that temperature for a particular time or hours. So, after the first person takes bath, it'll keep the hot water ready until the timer goes off. e.g., I set it to give me 65C water at 6:15 am and keep it at that temperature until 9 am. So, it automatically calculates the time it needs to switch itself on, based the water temperature and provides 65C water at 6:15am. It shuts off at 9am, when I leave for office. Memory is retained during power loss and it'll simply move to higher power band (upto 3KW) if it has less time available after power resumes. I found that in other models I could only set the start time and the rest would be a guess work.
Which model have you shortlisted from AO Smith? I've mentioned about my haier geyser because it falls in the same hi-end category as AO Smith.