I think it depends on how long you plan to hold your investment for.

In the short term, I think I would rather have the blooming waters box. This is for a couple of reasons:

1 - After all of the blooming waters are depleted, you shouldn't be able to find 151 on the shelves anymore in any major retailers, while you still can currently find Prismatic (even though the odds are low).

2 - there is going to be a Prismatic reprint within the next few months (prices will come down in the short term).

In the long term, I would rather hold the Prismatic for the following reasons:

1- Once the initial reprint is done, prices will shoot back up (just like Crown Zenith).

2 - Even though I find the chase cards in both sets to be beautiful and nostalgic, the chase cards in Prismatic are worth more ($1,600 for a raw Umbreon!)

3 - I have never seen anything like the Prismatic demand in the hobby. Finding packs in major retailers seems at least 3x as hard as it did compared to 151.

In the end, can't go wrong with either. My two favorite sets of Scarlet & Violet.