Even though Murtagh and Thorn were victorious in their battle against Oromis and Glaedr. I would argue that the Glaedr's bitting off the last three feet of Thorn's tail renders both Thorn and Murtagh more vulnerable, of the two I would argue that Thorn is currently the weaker of the two. Thorn lost the last three feet of his tail, the loss of this portion of his tail diminishes both Thorn's offensive and defensive capabilities. 

Offensively, the distance and range  with which Thorn can employ his tail as a striking weapon akin to a mace, club or flail in an attack wthether the attack is a frontal or posterioiral one has significantly decreased.

Conversely, from a defensive standpoint, the wound could also weaken Thorn's abilities to defend  himself in several ways: One, Thorn's loss of striking range reduces his ability to use his tail as a main gauche, buckler or shield like device with which to parry attacks from behind. Two, the loss of a sizable portion of his tail adversely affects Thorn's ability to execute as turns readily and rapidly,  Three, regardless of the healing process Thorn's wound may cause Thorn to have a decreased range of motion within his tail, as well as a more limited amount of control over what remains of his tail even after healing.

In additon Thorn's recovery may be complicated by the high degree of blood loss that is likely to have result ffom such a wound when taken in conjunction with exhaustion inherent in facing so formidable a pair of opponents as Oromis and Glaedr were in battle. By extension Thorn's vulnerability will place Murtagh at a greater disadvantage because, he will have to extend himself beyond his normal limits to protect Thorn from an attack directed posteriorially, I also think that if Murtagh attempted to heal Thorn alone without Galbatorix's aid that the effort would weaken him to such an extent that Eragon may able to overpower him