This prompted me to read a bit on this topic since I do not really know much about spark.
It seems that one of the core differences is the duration for which you need to trust someone to act honestly.
On liquid you need to continually trust the federation to be honest. At any time in the future they could theoretically collude against you.
On spark you need to trust them only at the time of the transaction to delete your keys. If they act honestly at the time of the transaction then in the future they can not collude to take your funds.
I am no expert in either but this is my quick read.
