resto staff heavy attack recover 10% max magika at the end of the heavy attack, this is a feature of the resto staff (if you check the dps resto staff heavy attack actualy deal less damage than spamming light attack but have the magika return to compensate for that while other weapon generate greater dps from heavy attack than spamming light attacks)

and not... this doesn't smite dark exchange a tiny bit... the most important point is that in order to recover magika with the resto heavy attack you have to COMPLETE the heavy attack... on the other hand you may interrupt dark exchange after the fist tick if you are in need of just enough magika for a single heal and you need it fast... at the same time Dark exchange with a 2 second channel recover as much magika (if not more) of a full lenght resto staff heavy attack while ALSO healing yourself (and the damage from the resto heavy attack is almost null... and please don't take out the argument of the passive that restore HP with resto heavy attack... that passive only restore 30% of the LAST TICK damage... that is to say it amount to around 10% of the damage of an heavy attack from any other weapon... just laughable)... the stamina consumption component is also greatly reduced by the morph dark conversion that lower the consumption to 12% stamina per tick making it enough low that unless you have to often break out of CC you will not even notice such consumption... so it may be a less functional alternative form of magika recovery... but still dark exchange fill this role better... (or to me more accurate it fill the roll in a more reliable and contrallable way) so unless you are really tight on skill slot as a sorc you really want to use dark exchange rather than rely only on resto heavy attack (using it every now and than when you get the time to use it may still be usefull... but if you need to get your magika back ASAP DE is way better than channeling heavy attack)

by magika restore I suppose that you'r talking about magika regen... if so you can't put it on armor (except for specific set bonuses) enchant for magika regen are for jewels only... and you probably want magika cost reduction there rather than regen... also regen soft cap pretty easy... and even with a 90+ regen (that is already above soft cap at VT4) you will only get that much magika every 2 second... while your spell cost around 200/300 magika (with full light armor and cost reduction passives) and you often want to cast them every second... so you recover at best 50% of a spell cost every 2 second... so 25% of you magika consumption when you really need it (more like 15/20% actualy)... and you can reach this much magiak regen with just light armor regen passives and racial passive... at best one single item with magika regen to get over cap with easy...

if you were instead talking about the Warlock set (recover XXX magika when under 33% max magika once every minute) than I'm not sure how much good that set actualy is... that is because it only comes in form of chest, neck and ring, all those item can't be enchancted (the have their own enchant anc can't be overwritten) and in order to get the set bonus you need to equip at least 2 jewel (2 ring or one ring + neck) and those have all magika regen as their enchant... if you consider the amount of cost reduction you cold get from those same slot, the amount of spell you cast per minute... and confront the actual reduction you get with the magika recover from the set... they cut quite close if not in favore of the magika reduction (especialy if you use lot of low cost instant spell rather than high cost long cast spell)... but I still have to actualy make accurate math on this to get the exact numbers (it may be warlock a little above or magika reductiona little above... but I don't expect to see a large difference in the 2 value...)

hope this clarify a bit ^^