By faster I'm referring to statistical averages in terms of which mode has the shorter average half-life. Intuitively I would think Beta decay because electrons and positrons have much lesser mass than alpha particles and are emitted from unstable nuclei at speeds of 95% of the speed of light.
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$\begingroup$ Side note: alpha decay happens nearly always with 5MeV, beta can happen with any energy. But both can have half life between femtoseconds and trillion years. $\endgroup$– peterhMar 1, 2017 at 0:52
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The answer depends on the individual isotope. Strictly speaking the half-life is of the original isotope, not of the mode of decay, but your question can be changed into one where if both $\alpha$ and $\beta^{-}$ occurs, which mode is more frequent and so implicitly happens faster?
Even here it depends on individual isotopes. For example, from the radium or uranium series:
- ${}^{218}_{\;84}Po$ sees $99.980\%$ $\alpha$ decay and $0.020\%$ $\beta^{-}$ decay so the $\alpha$ decay might be said to be about $5000$ times more frequent
- ${}^{214}_{\;83}Bi$ sees $0.021\%$ $\alpha$ decay and $99.979\%$ $\beta^{-}$ decay so the $\beta^{-}$ decay might be said to be about $4750$ times more frequent
I do not think there is a sensible way of taking an average over all radioactive isotopes, unless you measured their global natural occurrence and compared the total rate of $\alpha$ and $\beta^{-}$ particles being emitted at any particular time.
