(1) According to this opinion, the Korban is more strict because it possesses a higher level of holiness.

(2) The Gemara stated earlier that lashes are more severe than a Korban because the Gemara was considering which is a more severe punishment. Since a person would rather suffer the punishment of having to bring a Korban than the punishment of being lashed, it follows that the Korban is considered a lesser punishment than Malkus.

In contrast, Tosfos is considering which is the more effective atonement. Since lashes are a purely mundane procedure without any content of Kedushah, it follows that the atonement of a Korban, which possesses Kedushah, is more effective than that of lashes.

Kol Tuv

Dovid Bloom

1. Here is a different answer to your question. The Gemara in Berachos (61b) states that there are some people who prefer their money to their body. Accordingly, for some people it is a bigger punishment to have to bring a Korban than to receive lashes, because the expense involved in purchasing the Korban is harder for them than the physical pain caused by the lashes.

2. This answer is actually hinted at very tersely by the Rashash on the Tosfos here in Shevuos. He refers us to the Gemara in Kesuvos 32a. The Gemara there says in brief, "You might say that money matters are more lenient...." In other words, the Gemara there is in doubt about whether a monetary punshment is considered more severe or whether a physical punishment is considered more severe. We see from this that the same discussion here in Shevuos (that the Gemara says that a person would prefer to bring a sacrifice and not receive lashes, while Tosfos writes that there is an opinion that a sacrifice is a more severe punishment than lashes) is the subject of a disagreement in Kesuvos (whether paying money is more severe, or lashes are more severe).

Kesivah v'Chasimah Tovah,

Dovid Bloom