Both cities have cycle infrastructure and long distance cycle routes outside of the city. Therefore the plan it to buy a cheap bike in Tallinn (or hire a bike in both cities). I would be happy to spend a week + for both locations. Getting a ferry from Estonia to Finland takes 2 hours, and I potentially would only need to take one ferry. The flights are good value on Ryan Air from London Stansted to Estonia and vice versa.
Tallin looks like an awesome old city, in some ways similar to Prague (in the sense that they both have a medieval feel to it). Estonia is also the richest ex soviet country where lots of people speak English I think it’s a good location to visit. Helsinki, is a big city that seems to have a lot going for it and also Finland has more trees than any other European country by area – a random fact that might make it an interesting experience. Finland is often classed as northern Europe, maybe UK is as well not actually sure but would be cool seeing some of this.
Am looking forward to this holiday the most.
Edit: Holiday will be 11 days, and is planned for July.