Well, A lot depends on the condition of the truck and the brand name. But if you have a 2005 w/ 175,000 miles on it that means that the truck has been driven (on average) about 25,000 miles a year. Not uncommon, but it is high milage, and it usually means lots of highway miles, which are easier on a truck than city miles. Do you know the history of the truck?
Does the 98 have a carb or is it injected? How about the electrical system on it? Again, what brand and model? 100K on a 98 meams that it's had an easy life.. (about 14,000 miles per year)... but was it maintained regularly? My worry would be rust or corrosion due to the higher age.
Have you tried driving both vehicles? Brakes, lights, horn, heater etc. all work? All things being equal, I think I'
d go with the 98... but that's just me and I have a soft spot for 98's since I own a 98 Cherokee.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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