Yes, transaction speed and security are exactly the same across all Ethereum tokens and smart contracts. Slightly different transaction fees might apply for different implementations, but as long as known standards (like ERC20) are used, cost and especially speed and security should be pretty much (if not exactly) the same.
51% attacks can only be performed on the entire Ethereum Network at once, not specifically directed at one token.
To your question about stablecoins: Yes, if the future volatility is the same and they adhere to the same token standard, they would be equal in utility. But the thing about future volatility is that it is in the future and can't be reliably (if at all) be predicted from historic volatility. It is usually the one missing variable in the price-calculation for a financial option and is therefore subject to the price finding problematic.
Stablecoins achieve their stable price through two things:
- Having a reserve of value to start buying coins when the price drops a little.
- Users having trust into it. They will hold and balance the coin even though the reserve might be to small to pay for a total bank run.
The second part is actually the most important part, but also the only not-measurable part, so it is almost impossible to estimate the future volatility of a stablecoin and therefore to quantify it's utility.
Traditional fiat-currencies have no reserve and only have a stable value because of the trust of the general population.