Needles are fantastic for edgeguarding I agree. The actual projectile is the only thing I might really agree whole heartedly might make her better at. Also, Sheik's wall jump fair is godly and I am glad more Sheik's don't know how to use it like Amsah did when I played him.
Anyway, some points for Marth though:
-His Bair to fast fall or fast run to WD turnaround WD onto the edge normal allows him plenty of time to get the edge if he actually wants it. Fast fall -> Sweeping Bair off the edge is beastly for covering options; Sheik's is too but Marth's is a little easier to kill with and his UpB is conducive to responding to heroic recovery attempts if you mistime it.
-Sheik's Fair is better than Marth's Fair, but they are equally effective against offstage spacies (if you space Marth's Fair right). Also, don't forget about reverse dolphin slash having as much knockback (or more) than an Fsmash, and it's pretty much autodeath for most characters if he calls your low recovery with it offstage. Again, I think Marth's UpB > Sheik's UpB for lagless edgeguarding stuff might really swing it back to Marth.
-Marth's Dair is so disgustingly good for edgeguarding. The fact that it's one of the only true spikes in the game should speak volumes about how frequent you should try to utilize it; all of Sheik's kill moves are surivival DI'able so she has no equivalent. You can end so many characters stocks so quickly just by jumping on with Dairs or SH Dairs if they miss sweetspots. Also, Fair interceptions of recoveries often lead into Dairs if they miss the away DI on the Fair or Marth gets a sweet spot for combo'ing on the Fair.
-Also, besides that Sheik only really needs Fair/Bair for edgeguarding and both can be avoided by DI'ing into stage on a recovery/riding the wall with a Firefox/bird. Sheik has a lot of trouble with the latter option, where has Marth has Counter to make spacies UpB's and aerials from the edge recoveries extra dangerous.
-Also, charge Neutral B as a reactionary edgeguard. You would be amazed how many timings you can cover by just jumping off the stage (FH/SH depending on situation) and following them horizontally down while charging Neutral B. This can often hit them out of their jump/UpB when they not expecting at a distance from the stage dramatically hard to recover from. It's a fairly short animation (from release of B to end of lag) so it can also be used early to early timings and then released to grab the edge and cover different options.
-If all else fails, overall holding the edge with Marth ***** characters with laggy recovery for the sheer reason that he always at least has the option to get them to land, then full FD on into turnaround tipper (if possible), Utilt (at higher %'s vs Floaties), Grab (Dthrows reset Marth to make up for botched edgeguards often, Uthrow leads to crazy free stuff at mid to high percentages on almost all of the high tier), SH/FF Dair (on grounded opponents can often lead into hitstun bounces into Fsmash/Dair again/Dolphin slash etc) regular Fsmash/Dolphin slash (again gives Marth stage control and a new chance to edgeguard at the other side of the stage).
Basically what I'm saying is...while Sheik might seem like she beats Marth in certain aspects of edgeguarding, Marth's overall amount of edgeguarding options give him crazy amounts of mixups, all of which can generally cover most if not all options and still leave him room to get back into good edgeguarding positions/stage control and the presence of certain moves (counter/Fsmash) often encourage an outstanding weariness of using half of certain characters available options.
Sorry for the wall of text. Hope this helps...someone. haha