Phoenix vs Exile

Neither of them necessarily makes or breaks the build, but Phoenix is only worth the cost for some builds. The Life Tap on Exile is too useful to give up, especially given how expensive Phoenix is. Especially if you are smiting somewhat regularly, where life steal doesnt work so Life Tap is critical.

The thing is, you probably arent going to notice any meaningful difference by swapping, you are just using 4 high runes. If you have the runes to spare, then sure, have a play. But if you arent sitting on piles of high runes, probably focus on other things, like a good roll Grief or something

I may be biased but..Exile hands down.

What makes Phoenix BiS (for DPS) is the massive ED it offers which no other shield does. As a result, for a BiS zealer/smiter, switching to Phoenix is somewhere around a 15% raw, direct DPS increase, give or take. However, that is solely on the raw DPS side and beyond that, it only offers rather minor additions.

Exile on the other hand is hands down the single best thing ever for defensive purposes imo. The Life Tap proc chance is comparatively massive and you generally never need to worry about it again once you get your hands on one, making the entire build significantly more relaxing and tanky to play (especially vs. ancients/ubers/..). The boost to raw defense is equally massive. And much like Phoenix, you get some other minor additions as well.

Comparing those two, the thing is that Phoenix in my experience sounds a lot better than it actually is, in part simply because a smiter/zealot tends to come with fairly solid crushing blow. As a result, more and more of your damage shifts away from "direct raw DPS", in turn making that 15% gain with Phoenix quite a bit less when looking at the overall damage output.
Additionally, if you're otherwise in full BiS, smiter/zealot per-hit dmg is so high that that is regularly not a main concern anymore anyways.

Now, IF shield-ED worked like weapon-ED and actually directly increased the base shield Smite damage of the shield instead of being rolled into other ED sources, then yes, it'd be absolutely insane as you'd be looking at flat out doubling your damage (if not 5xing, depending on how that'd play out with Holy Shield). But it doesn't. So you remain at ~15% raw dps that you really barely notice at the cost of a drop in survivability and "relaxation thanks to reliable early Life Tap procs" that you very much do notice.

Ultimately though, both are solid choices so that, too is largely a matter of preference.

I am team Phoenix I need the adrenaline from only having 5% Chance for proccing Life Tap from Dracul's Grasp when doing Uber Tristram or Colossal Summit