Let's take a look at the biggest Biotech ETFs, as this was one of the market's top sectors for several years and has recently undergone a pretty strong correction.  

As you can see on the following table, there are currently only 2 pure sector play Biotechnology ETFs that trade over 1 million average daily volume (XBI) (IBB) — but there are 5 that have options trading available (XBI) (IBB) (FBT) (BBH) (SBIO). Caution on the option liquidity on some of these, other than IBB — use limit orders to avoid potentially bad fill prices. We've omitted Inverse & Ultra ETFs from our analysis.

Table

Source:  etfdb.com 

You can see from the Year-To-Date (YTD) returns that the ETFs within this sector differ greatly — the gain/loss this year ranges from -7.6% to +8.5%.

Holdings/Expense Ratio:

The difference in these ETFs largely comes from the holdings (and expense ratio) — here are the expense ratios of these ETFs currently (holdings and expense data from Yahoo Finance):

XBI = 0.35%
IBB = 0.48%
FBT = 0.58%
BBH = 0.35%
SBIO = 0.50%

For comparison, Biotech Mutual Funds range from 0.78% to 2.18% expense ratio, and many of these have Loads/Feeds that add additional costs — in general, ETFs are a better bargain for active investors than Mutual Funds.

Let's look at the top holdings and diversification of these ETFs, to see similarities and differences:

Top 10 Holdings:

XBI has 11.8% of assets in its Top 10 Holdings — (RDUS) (MYGN) (JUNO) (INFI) (MDXG) (EXEL) (BIIB) (EXAS) (ABBV) (ALXN)

IBB has 60.3% in its Top 10 Holdings — (BIIB) (GILD) (CELG) (AMGN) (REGN) (ALXN) (ILMN) (VRTX) (INCY) (MYL) — the first 5 of those are each around 8% to 9% of total assets.

FBT has 39.3% in its Top 10 — (MYGN) (GRFS.BC) (QGEN) (INCY) (Q) (TECH) (NKTR) (AMGN) (CRL) (REGN)

BBH has 73.5% in its Top 10 — (GILD) (AMGN) (CELG) (BIIB) (REGN) (BXLT) (ALXN) (ILMN) (INCY) (BMRN) — top 3 are 11% to 16% each