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It somewhat depends on your game situation.

If you have just that one weapon, need to upgrade it urgently, and are likely to have enough coins the next time you need to upgrade a weapon, then clearly the 30% option is better: It gives you a 30% probability of upgrade, while three 10% attempts give you a probability of $1-(1-0.1)^3=0.271$, that is 27.1%. Of course with the 30% strategyyou will lose 300 coins for sure, but by assumption, that is your lesser problem.

On the number of weapons you can upgrade per 300 coins, both strategies are equal. The 30% strategy gives you on average 0.3 upgrades for 300 coins, while the 10% strategy will give you on average 0.1 upgrades for 100 coins, which again gives 0.3 upgrades per 300 points. Here the lower probability of getting an upgrade at all for the 10% strategy is offset by the fact that you have a good chance to have money left to upgrade another weapon. If you are really lucky, you may even upgrade three weapons with those 300 coins, although that is pretty unlikely, with a probability of 0.1%.
The difference in the two strategies in terms of number of weapon upgrades is that with the 10% strategy, your distribution is stretched further (you have the ability to get more upgrades altogether, but also an increased probability of getting fewer upgrades).

However I guess with upgraded weapons, you are also able to obtain more coins in the future, so maybe the question is which strategy allows you to get your updates as *early* as possible. In that case, the 10% strategy is the better one, as that way you can upgrade as soon as you've got 100 coins, instead of waiting for getting 300 coins, while maintaining the same average number of upgrades as described in the previous paragraph.

Of course all this assumes that all upgrades are of equal value. If you have one weapon where an upgrade gives you a massive advantage, and another one where the upgrade only gives you a marginal advantage, it might be a better idea to put your 300 coins in a 30% chance upgrade of that valuable weapon, instead of sacrificing some of that for a possible additional upgrade of the weak weapon.