The main advantage of IWM over VTWO is greater liquidity.
At the moment, Yahoo Finance is showing average daily trading volume (in the last 3 months) 91,651 shares for VTWO and 24,218,219 for IWM.
Another advantage of IWM is the availability and liquidity of options - it is among the top ETFs by options volume. VTWO technically does have options available, but the range of expirations and strikes is much narrower, while the bid-ask spreads are extremely wide.
As a result, IWM can be preferred over VTWO if you trade more frequently and/or with bigger size, or if you want to use options (e.g. sell covered calls against your position). Additionally, some prefer IWM out of habit, for the mere reason that IWM has been around since 2000, but VTWO only since 2010.
If you are a small investor looking to buy and hold for long term, you might prefer Vanguard for its lower expense ratio. Generally, the longer you intend to hold the position, the more expense ratio matters. The shorter your time horizon, the more you should focus on liquidity and transaction costs.