What is the difference between perpetual ETH and isolated margin trading?
Perpetuals are an explicit contract type with no expiry. Margin can be functionally similar, but refers specifically to accessing more funds. Also, trading 1x leverage on a perpetual doesn't use 'margin'. On dYdX, perpetuals also offer higher effective leverage (compared to isolated margin) with up to 10x. Perpetuals are derivatives, so they trade synthetic assets that are settled in USDC/ETH depending on the perpetual. Margin trading involves the actual asset.
When to use Perpetual vs. Margin
They can be used for similar needs for certain traders (for example, increasing exposure or hedging), but note that margin trading has the ongoing cost of interest for borrowing while perpetuals have a funding fee (so they are different calculations as shown on the dYdX website). Besides higher leverage, another basic example where perpetuals can be helpful vs. isolated margin is for cost-efficient hedging (such as buying a perpetual to hedge out short exposure on dYdX or another venue, or selling a perpetual to hedge out long exposure on dYdX or another venue).
If you're holding ETH in your wallet and want lower trading fees, access to higher maximum leverage, and clear PNL (given in ETH) after shorting or longing, then the Perpetual is probably a better choice.
PNL for Perpetual vs. Margin
For Perpetuals, PNL is displayed in ETH, but in USD terms for Margin trading so many users will find PNL to be clearer for perpetuals. Margin isolated positions also show clear PNL, although in % instead of absolute ETH.
Are Stop-Limit trades available for Margin and Perpetual?
Yes, Stop-Limits are offered for all dYdX markets.
Does liquidation work the same way with margin trading and perpetuals?
Liquidation works similarly, the liquidation ratio for perpetuals is lower at 107.5% vs 115% for margin.