Lapras or Walrein

How is your stardust looking, first and foremost? You're probably looking at dropping ~100k on one of the two (more if you plan on maxing one out). Plus, your Rayquaza team is good as-is, and you won't get quite as much mileage with ice types for upcoming raid bosses.

If you're set on powering one up though, between the two, I would probably pick the Spheal if you have enough candies (200+) to evolve it and power it up. It's best moveset is FB/B, which is the same for (non legacy) Lapras. However, Walrein has the higher attack power between the two, which should net you slightly higher DPS compared to the Lapras alternative while also adding some variety to your team.

I wouldn't power up any pokemon for specific legendaries as as soon as they go they are useless, I would save the stardust and evolve lvl 35 spheels and shelders for rayquaza, as a family of 3 we can do any rayquaza moveset with over 100 seconds left and non of us have put any stardust into rayquaza counters. If you want to power up the spheel as a trophy then do n=but in battle IVs make little difference

I would honestly power-up neither of them, given you are not in shortage of needing immediate Ice mons. Also, given your small buffer of expendable dust.

Walrein? Seriously that thing has a creepy face. I dex'd it then binned it. However, this is the more practical mon if you really really must.
Lapras? Awesome mon, but a bit lacking in the firepower department. It's a shame the 2 bar move (higher flexibility) is now legacy.

I would choose Walrein of the two, but reading your comment, with the amount of dust you have I would seriously consider just leaving both as they are and saving your dust for something with a bit more longevity. We only have a little more than a week of Ray left and at that point ice-types pretty much fade out from the meta. Lapras is a better defender out of the two if that's something you're looking for, but neither is going to be good long-term because of Mamoswine in gen 4.

A big Lapras fan, I still have to agree with the overall consensus above, hold your dust.

If you already have two Lapras, I'd say no on that one. Not every Pokemon with good IVs will need to be powered up, especially since you'll be getting more examples of such over time--and new species will want your attention.

The Spheal is far too low level. They're everywhere, and you can find a high-level candidate for far less investment and for very little drop-off in effectiveness.

I'd recommend sitting tight.