Hi mate,
I have a huge respect for both these groups and, with either, I think you would be in good hands, regardless of where you ultimately decide to pursue your discernment.
I only have one brief experience of the FSSP - their sung mass on Maundy Thursday in Edinburgh. I have told you about it already, but I was very impressed by the quality of the liturgy, the number of priests and assisting ministers they could boast, and by the highy quality of the small group (schola?) of lay people who assisted the celebrant in singing the mass.
Although the attending congregation that day was small, it was worth noting for the broad appeal the congregation represented. (I expect their congregation is bigger for normal Sunday mass, and you-tube confirms this).
Ages ranged from people in their 20s, to probably in their 70s/80s. The younger people tended to be in couples (doubtless married), the older people mostly on their own (likely widowed). But there were exceptions (including me lol). I noticed one young gentlemen of Chinese ethnicity also, which was remarkable as Scotland is not the not ethnically diverse place, let alone the minority Catholic community (Incidentally, I have noticed Chinese-Scots present at the Glasgow SSPX mass too). There was also a young Gentleman who used a wheelchair, showing that accessibility for all is considered. This broad appeal is to the great credit of the FSSP.
Before the mass, a priest came among the pews and asked a young man to help with a procession (holding a canopy) and gave instructions. They obviously knew each other and seemed to be friendly, which suggested the FSSP cultivate good close relations with their flock.
I have no direct personal experience of ICKSP, but have always had a favourable opinion of them, mainly due to their work when they:
- In 2011, they took over a closed Church in Wirral, UK, at the request of (the excellent) Bishop Mark Davies. They expected 400 people for their inaugural mass, they got just shy of 1,200. This is it: domeofhome.org/
- Bought a Church - a real gem, destined for secular use (a pub or whatever) - in Limerick, Ireland which had been lying empty (ex Jesuit) and made it into a traditional chapel.
rorate-caeli.blogspot.com/2012/08/you-report-icksp-given-church-in-ireland.html
I think they are both first class examples of the Catholic priesthood, and think it really comes down to your own preference more than anything. Language may be an issue - I
think the FSSP is mostly French, the ICKSP mostly Italian (?). (I may be wrong).
Also consider where you might like be able to minister: for example, the FSSP is present in your home country, but ICKSP is not. (Obviously I would urge you to minister here in Scotland! I would come to your parish, even if no-one else did lol).
Why not make contact with both groups and see what your heart, what God, tells you?
The FSSP have an advantage is that they are only a short drive from you (Edinburgh) and so you can meet with them in person easily enough.
Cheers
GW