It depends greatly on the campaign, the level of the PCs, and house rules of the DM. Personally, I have used several house rules to make PC death a greater worry than magic item loss, but from what I have seen and heard of other DM's games, loss of items is typically more worrisome to PCs than loss of life - a rather sad fact of 3.x e, I think.
Permanent level loss is treated oddly in 3.5e. I instead use the 3e version. When a PC levels up, I first write out on a separate sheet of paper their arrangement of skill ranks, feat selection, abilities, etc on another sheet of paper. If they permanently lose a level they go back to what they had before, rather than get a -1 to all rolls, checks, etc until they return to their former level. While requiring more paperwork, this has worked very well both for myself (in keeping track of things and a sense of believability for the setting) and for the PCs (some of whom developed interesting backstories, whereby their character lose a few levels then chose different classes, skills, etc upon re-leveling, choosing to best make up for their former loss. In one example a 7th level cleric lost four levels, then chose to level up in Ranger (favored foe: undead) due to his character seeking vengence against the undead that had so weakened him. We later cobbled together a PrC that granted some turning and favored enemy advancement for his character.
So, in a typical RAW game, I would rate the selections as:
1) Item Loss
2) Level Loss
3) Death
Loss of items is an immediate concern, especially beyond early levels, as they - often more than class levels - dictate the power and survivability of a character, whether PC or NPC. No matter how high the level, some creatures have DR that requires special advanced weapon types, or have attacks that can slay any not wearing appropriately magical armor, and so forth. Considering the number and the diversity in powers of the magic items a RAW campaign setting suggests a PC of this or that (beyond low) level should have, it can be troublesome and time consuming for both player and DM to restore the PC to an appropriate level of power for their level.
Loss of level is an aggravating concern, as it imposes a -1 and shoehorns the player into whatever levels they priorly had even if they do not want them anymore or they make no thematic sense. Beyond mid level it can be restored rather easily once the current encounter is over - or at least within a few days at worst in a RAW setting. Thus it is more a short term aggravation than the long term hindrance that losing multiple level-appropriate magic items.
Death, in a RAW campaign, is equal to permanent level loss. Once around mid level or so, it is a short term aggravation rather than the longer term aggravation and hindrance that loss of magic items entails.
In a mid or higher level campaign, a quest can be thrown for a loop and significantly sidetracked by loss of most magical items. Death and level loss, however, merely delay the quest for the day it takes for the local cleric to pray to their deity for the appropriate restorative spell. They might attempt to continue adventuring without all their magic items, but the RAW make any encounter significantly - and sometimes even impossibly - harder, thus making death, and perhaps even TPK, a far greater likelihood. This is not even taking into account the fact that the PCs themselves will feel far more insecure, vulnerable, etc at the thought of continuing without the items that give them a chance for survival and success in their current quest.
This is one of the reasons I've considered changing over towards Iron Heroes or some similar system for a while now.