I wouldn't put too much stock in explanations of "loyalty," something that we have very little evidence for and which doesn't do a very good job of explaining most things. Besides, Pompey's troops at Pharsalus were not his veterans, but had been raised by his senatorial allies from the east. You'd have to ask Pompey why he thought he should do one thing rather than the other, but already in antiquity observers were offering their own opinion. Plutarch thought that Pompey, as obsessed about his own dignitas as Caesar ever was, was wounded by the dolor of repeated insults and taunts on the part of his allies, who poked fun at his pretensions to kingship (Plutarch quotes Domitius Ahenobarbus as calling him "Agamemnon") or his inability to protect Roman provinces and Roman lives (Plutarch says Favonius made fun of Pompey for being unable to hold Italy). Plutarch believes that Pompey's concern for his own dignitas led him to take on the challenge as a means to silence his detractors and win eternal fame--he mentions a dream Pompey had the night before the battle in which he was applauded while entering his theater. In any case, by Plutarch's account, it's not totally clear that Pompey had any other choice. Far from being eternally loyal to Pompey, the troops were being canvassed by the various senators accompanying the army, who were preparing for their various bids for magistracy and hoping to gain the troops' votes. Moreover, the senatorial leaders initiated a council in which they all swore to meet Caesar in a decisive battle--Labienus, Caesar's former legate and Pompey's cavalry commander, was the first to swear the oath, which surely had some effect in forcing Pompey's hand. In the Caesar Plutarch also considers the cost of a war of attrition. Cato in particular, he says, urged Pompey to fight Caesar decisively because a prolonged civil war in which both sides were just trying to weaken the other by constant deprivation would be catastrophic to the Roman citizen population. Also in the Caesar Plutarch mentions that Pompey's own troops were desirous of a decisive battle, and in particular his cavalry, on whom he was relying. In the de bello civili Caesar attributes Pompey's actions to his overconfidence and the overconfidence of his troops, who had defeated his army at Dyrrachium and believed that the time was ripe for a single decisive engagement. Appian believes something similar, sort of between Plutarch and Caesar. Appian thinks that in the council at which Plutarch says the officers of the army swore to meet Caesar in battle Pompey received conflicting advice. Afranius urged him to continue harassing Caesar and in particular to use the fleet, which Caesar could not match, to wear him down. Others, however, argued that if Caesar was so weak as all that his army would collapse soon and he should be pressed with battle as quickly as possible--essentially, this camp considered the march to Pharsalus an extended pursuit. Appian says that Pompey accepted this advice partly out of concern for his dignitas but also because he wanted to capitalize on the high morale of his troops and to protect his own flanks--he had eastern allies that Caesar might attack if he didn't pursue him closely enough, and Caesar might get between him and Lucius Scipio's force in Macedonia, cutting him off from the east. I expect personally that if Afranius did suggest that Pompey rely on the fleet in particular that cannot have been the greatest of arguments, as the Pompeian fleet had failed to stop Caesar from crossing into Greece despite their overwhelming numerical and technical superiority and Caesar's total lack of warships.

Who you believe is up to you, but I think all these accounts point really to the same problem, a cauldron of issues, personal and military, that appeared to have a ready solution, one that didn't seem particularly risky at all.