Given the choice between an index tracking fund or ETF that tracks the whole (“total”) of some particular index or market, or the top X companies in that index, and knowing nothing else, what would your assumptions be about them?
Is one going to more stable than the other? Probably earn more dividends? Anything else? Or is there nothing you can guess about possible differences between them?
makes me think of this article I read yesterday. Its about the concept of an Index and the level of detail you can get into.
I think its highly subjective. Would you call FANG and index? What about FAANG? Or FAMGA?
Bit hard to say until “top X” is defined - “top” implies “best” I guess. But even when it is defined, best is what every fund or active manager is claiming to pick, and we see that many of them are wrong.
So the assumption might be that compared to all of an index/market/group of securities, a selection might offer increased volatility, but we probably don’t know if it will offer better or worse performance 
If the cost of the “selected” fund is larger than the comparable “all” fund, we might assume that its performance net of fees will be worse.
That is passive investing orthodoxy: buy the most diverse and lowest cost investment you can, ie the “all” fund. The alternate approach is to simply pick the securities that’ll perform better than the market as a whole, easy! Though it turns out that’s easier to say than do 
Thanks, I think that’s a good generality.
The specific example I’m thinking of, though, is Vanguards S&P 500 ETF (0.07%) vs their “U.S. Equity Index Fund” (0.1%), which tracks S&P Total market index. So, in this case, the fund/ETF tracks the whole of their respective index, but the indexes differ on if they are total or top X. (I think I phrased my question poorly to match this scenario.) And it happens that their ETF on the top X is cheaper.
0.03% difference in annual cost isn’t going to make much difference unless your holding is large. If it was me I’d also look at something with more geographic diversity - an “all world” etf - even though the annual cost will be higher. Like VWRL from VG’s stable.
(But that’s me. Your aims, needs, attitude/appetite to risk, investment horizon, age, family, hopes, waking financial nightmares and other circumstances may be different to mine )