'best' in audio is very much like the 'best' car. i feel that my 2001 mitsubishi eclipse is the best car, but am sure that many, many people would disagree..
the cards your looking at are 2 completely different categories.
the x-fi surround pro is a consumer-level usb sound card. i had one for a while, and it has a turn-on pop, so i don't typically recommend it, as there are many other solutions that don't have this problem.
the terratec is a pro-audio grade sound card, and as such, it has software that is intended to be used in professional applications-- so getting all the audio channels to work could be problematic without requiring extra pro-audio software.
and the other thing i am not so sure about is that it has a power input for a add-on power adapter. while some devices don't require this, many do require a power input to work. in my brief search, i didn't see what the power requirements are for this device-- if it requires 12v or less, there is a good chance that it can be powered in-car, but could require extra parts.
some other things-- you say you want to use RCA connections to the amps. i can assure you that a 3.5mm-to-rca cable adapters will not audibly hurt your sound quality.
technicality: yes, any connection, or splice will adversely affect the audio signal, but when using decent quality cables (mid-grade cabling), it would take many, many, many, connections to get to a point where the audio signal will audibly change... and this applies no mater what type of cable connector you use.
what do you need the sound card to do(multiple outputs? audio routing?)? i might be able to locate some suggestions.