Perhaps depends on how you value the different skills of a fast bowler.
Lillee was probably the most complete quick in history, as he had all the basic values of a fast bowler and was amazingly accomplished at all of them. Marshall was very versatile as well, but wasn't express pace and didn't have the height usually associated with quicks.
Marshall was more successful in the literal sense, as indicated by his superior average and strike rate, but it could be argued that Lillee faced higher quality batsmen all round, as he had to face the West Indies and did so with success, and Marshall played more through the 80s in a weaker period for world batting. Lillee could also never hide, at least not after Thommo's decline.
Marshall was part of a series of legendary pace attacks and distinguished himself through his work with other great bowlers, while Lillee was the entirity of the Australian bowling for most of his career, and performed well with that responsibility.
Marshall performed in all countries in all conditions with distinction, while Lillee never proved himself in the subcontinent, albeit mostly for a lack of opportunities. Lillee perhaps played on more docile wickets more often though, given his home conditions in Australia compared to the famously pace-friendly West Indies of the time.
Marshall was consistently deadly through the middle of his career, only ever dipping below his high standards at the start and finish of his time in test cricket. Lillee was more up and down, but had to deal with occasionally being overbowled, and a back injury which would have ended the career of most bowlers. Lillee also missed a significant period of international cricket at the very height of his powers due to World Series cricket, where he outperformed all other bowlers.
Marshall was the more reliable of the two bowlers, rarely ever suffering a bad game or a thrashing at the hands of any batsman. Lillee was perhaps the more devastating, putting in some of the most famous spells of his time against some great opposition.
Lillee perhaps adapted better to the differing requirements of ODI cricket, and has a much superior record, but played far less of it. Marshall was a better batsman by some distance.
Both bowlers were undeniably great, and either side of the above cases could be argued for each. Both would undeniably make the all-time XIs for their respective countries, the World XI for any period in which they played, and would push for selection in an all-time World XI too.