I don't think US high school is harder than the UK, just different (if anything I'd argue the content might be a little easier, but the way it's assessed makes it harder to get good results - you're often more assessed on how well you can take particular types of test, rather than how well you know the material).
Since you're of that age I'd probably suggest going in as a freshman for two reasons as a result:
a) it's a totally different education system, so it will give you some time to adjust to the new style and format of teaching and learning there. The material will probably be fairly accessible or even easy for you regardless of which year you go into, but getting used to the multiple choice standardized testing format will take some getting used to (as will being graded on weekly assignments/quizzes/etc rather than mainly a final exam as in the UK).
b) socially, it might be easier to integrate into the new freshman class than joining an existing sophomore cohort. This in turn may help you adjust to the academic side of things by making the social side slightly less daunting.
It won't matter whichever one you go into when you apply to colleges, unless you do worse in the end as a result of your choice (which would disadvantage you). Another reason to go with freshman rather than sophomore imo.