Takeaway: 
Empirical evidence shows demand‑side policies reliably raise output in the short and medium run, especially through high‑multiplier spending, while supply‑side tax cuts have weak or statistically insignificant effects on investment and growth in most real‑world studies. The evidence overwhelmingly supports the idea that demand matters, especially when the economy has slack.

Below is a structured, evidence‑driven comparison using the sources you triggered.

📊 Empirical Evidence: Demand‑Side vs Supply‑Side

🟦 1. Demand‑Side Evidence: Investment Responds to Demand Conditions

High multipliers from public investment and welfare spending

A World Bank study of 129 countries (1980–2019) finds that a 1% of GDP increase in public investment raises output by 1.1% after five years.
This is strong evidence that demand‑side fiscal expansion increases long‑run output, not just short‑run consumption. Open Knowledge Repository

Keynes–Kalecki mechanism confirmed

The Springer research summary notes that firms base production and employment decisions on expected effective demand, not on supply‑side assumptions like Say’s Law.
This supports the empirical observation that demand determines investment, not the other way around. Springer

Modern macro (HANK models) shows demand shocks transmit strongly

The Reserve Bank of Australia’s research paper shows that in heterogeneous‑agent models, demand shocks propagate powerfully, especially when households are liquidity‑constrained.
This aligns with the idea that welfare transfers to low‑income households have large multipliers. Reserve Bank of Australia

🟥 2. Supply‑Side Evidence: Weak or Inconsistent Effects

Tax cuts for high earners show limited growth effects

Investopedia’s summary of supply‑side theory acknowledges that its claims are theoretical, not strongly supported by empirical data.
Supply‑side policies assume growth comes from lowering taxes on capital and high incomes, but the evidence base is thin. Investopedia

Historical performance is mixed

GovFacts’ comparison of supply‑side vs demand‑side economics notes that supply‑side policies have not consistently produced higher growth, and often increase inequality without raising investment.
This aligns with broader empirical literature showing capital‑income tax cuts rarely pay for themselves. govfacts.org

🧩 3. Synthesis: What the Evidence Actually Shows

🔥 4. Why Demand‑Side Policies Outperform Supply‑Side in Data

1. Firms invest when they expect customers, not tax cuts

Empirical studies repeatedly show that capacity utilisation and expected sales are the strongest predictors of investment.

2. Low‑income households have the highest marginal propensity to consume

This makes welfare spending one of the most effective fiscal tools for stimulating demand.

3. Supply‑side tax cuts often leak into savings

High‑income households save more, so tax cuts do not reliably translate into productive investment.

4. Public investment has direct and indirect effects

It raises demand immediately and increases productive capacity over time.

🧠 5. What This Means for Your Original Question

Supply‑side economics assumes investment is driven by tax incentives and long‑run profitability.
But the empirical evidence contradicts this:

• Investment responds strongly to aggregate demand,
• Welfare and public investment have large multipliers,
• Supply‑side tax cuts have weak or inconsistent effects.

In short: demand‑side stimulus is empirically more powerful.

If you want, I can now break down:

• the strongest empirical papers on multipliers,
• why supply‑side theory persists despite weak evidence, or
• a combined model that integrates both approaches.