Lappi and Suninen started driving WRC at almost same time (2 rallies difference) so they are the two most natural ones to compare. This year Suninen ended 9th with 89 points and Lappi 10th with 83. In stage wins it's 7 to 6 for Lappi.
There are differences for sure, but even last year at Toyota Lappi was going slightly backwards. He had good results in first half of the season but when Toyota upgraded the car and became clearly fastest in the second half of the season Lappi just moved backwards. Sure his move to Citroen was decided just before that and could have an effect. At Citroen he has the most extremely mixed record possible. Great pace in 3 rallies, and no pace + crashes in 10 others.
Suninen has a bit of an opposite trend with mediocre start and slowly improving (specially on tarmac).
Therefore right now I don't see a huge difference between them. "Future potential" is just speculation at this point. One thing they have in common (that imo comes down to experience) is the issues they have in rallies outside Europe. For last two years they are regularly among the last WRCs, minutes behind or crash.
This brings us back to my main point, which is not a Lappi/Suninen comparison: MSport should have one experienced fast driver and they did have one in recent years. (Evans in 2019, Ogier in 2017-2018, Østberg in 2016). Last time they lacked one was in 2015, which was quite bad year and first in a two year decline. As it looks now history might repeat itself.