Hey!
I personally wouldn't read it as one role necessarily being easier to get than the other - it is more likely that they either have additional vacancies, or think that your profile could suit both. In terms of whether you would have a better chance by switching, it is quite difficult to know without any additional information / context. Unless they have explicitly indicated a preference for which role they'd like you to apply for, there is no guarantee that moving to the new role would improve your odds. Because of that, I would focus less on strategy and more on which role you would actually be happier doing day-to-day, even if the difference between the two feels quite small.
If you are completely neutral, it is still worth taking a look at both job descriptions to see if one stands out even slightly - for example, in terms of the type of work you would be doing, the exposure you would get, or the progression opportunities in the future. You could also reply and ask for a bit more information about how the roles differ before you make a decision, as I'd say this is a completely reasonable request.
If after all that you still don't have a preference, sticking with your original application is absolutely fine. Alternatively, if they perhaps suggest that the new role might be a better fit, it might be worth following that advice.
Overall, I don't think there is an obvious right or wrong choice here (as frustrating as I'm sure that might be!), so I'd try not to overthink it too much. If you really can't decide, I'd just go with your gut instinct
