What about Silver/Warehouse? Is that a better alternative to these two?
If you're just trying to hit $5, then Warehouse/Silver has a slight disadvantage to Watchtower/Silver, as you will trigger the reshuffle on T4 assuming Warehouse doesn't miss, so it's possible your $5 purchase doesn't make it.
But whats the odds that you will hit $5 before turn 5 with Warehouse/Silver?
If Warehouse doesn't miss T3 or T4, you'll hit $5 for sure, but if you draw three estates on T3, or play Warehouse on T4, your $5 (and technically your Warehouse too if played on T4) misses the next shuffle. If Warehouse + copper is on the bottom, then it's a 31.8% chance to miss $5 (same as Watchtower+Copper). Warehouse+Estate on the bottom and Warehouse+Silver on the bottom have the same chances as Watchtower+Estate and Watchtower+Silver on the bottom.
58.33% chance your Warehouse misses the shuffle, as opposed to 16.67% for Watchtower.
59.7% chance your $5 purchase makes the shuffle.
Long story short, Watchtower+Silver is more reliable overall.
(Someone may want to check my math on this; may be missing some edge cases, but I'm pretty sure what's up there is right...)