They offer similar things and both revel is the same position, it's very much like for like to be honest. Ofcourse Moyes' will interchange them occasionally but I have no doubts that he has an idea of what his starting eleven would look like with everyone fit.
I'm just looking at it realistically, when Neville and Arteta are back they'll both be in the team, they are his first choice centre midfield. You've got Jack Rodwell in reserve, options outwide with Bilyaletdinov, Pienaar and Osman and Cahill and Fellaini further forward.
Heitinga, Neill and Hibbert at right back which means it's very likely that Neville will play in midfield. Oh and obviously this quote:
“There will be times when I use Tim in some games and Fellaini in others and not both together,”
“Once I get a squad, that will happen. At the moment, I don’t have a squad to make those decisions.”
Not for me really, granted Fellaini and Cahill share a lot of attributes but equaly there are many they dont. Im not sure both will or can be in the team together for balance but their not like for like by any stretch IMO. Both are capable in the box, have a good engine etc but Cahill never dropped as deep Fellaini and was never a consistent option to pick the ball up of the back four to turn over play. Equally Cahill always relied on his energy, tenacity and willingness in midfeild, he was a get stuck in, give it and go quickly type of player when he played deep. Fellaini can make yards up with the ball coming from deep in pocession, he is excellent at turning deep positions into the foundation for an attack by chooseing the right option for distribution, or carrying the ball - he has, although we have seen it rarely the ability to pick a Hollywood pass too. Your right in the sense i do think it will be either or - just for balance though.
Frankly, although im a fan of each, both Fellaini and Rodwell cant be considered nailed on first team starters, mainly because both dont influence the game consistently for 90 mins, they will in the future. Although Fellaini may influence the game for about 70 mins he rarely goes beyond that or impacts conistently wholly the way he will be able in seasons to come. Equally Rodwell presently may only impact for maybe 40 - 50 mins. Thats not a ciriticism and both probably have more talent then players who will possibly start ahead of them at times this season. Its just reality and reasonable given their ages and the context of their development. They certainly have huge roles to play this season, but neither has proven really they can provide consistencey in performing and influenceing a game for 90 mins in midfeild - never mind a season.
Which is were really the expierence of Cahill and Neville comes in - proven, effective talented above all expierenced campaigners they will start many games maybe the majority all fit - but i dont think its necessarily a definite, given both versatility and ability to play a number of roles and the talent of both Rodwell and Fellaini.
Im not a huge fan of Neville in midfeild, hes done well for a RB playing in midfeild but as centre midfeild play goes hes average. The plus points being 1) he facilitates Arteta (who safe to say is a must) as he is more effective with an equliser in beside him, 2) he protects the space in front of the back four allowing Cahill to support the striker and 3) hes a great leader. I wouldnt be dissapointed to have him back at RB to be honest which for me is his innate position - i think
Moyes plays him in midfeild to maximise the ability of the players around him and give a balance rather then any ability he has in midfeild, certainly not that of Rodwell Or Fellaini.
I think it may be a possiblity that a system could be divised that may negate the need for a holding midfeilder with Jonny H at centre back, based on the role he played stepping up into Rodwells position when we were attacking on Sun - a bit adventourous for
Moyes though over a season so cant see it as a reality.
To summerise though we have a decent blend of youthfull talent and expierence which will offer us fresh talented options in youth but consistency, relaibaility and trusted guile and effectiveness over the season. Like i say to speak in definites is to cut your nose of to spite your face, each will play a role thankfully this season.