Performance of the SRX300 is roughly the same as a SRX240H2 - so you performance experience will be the same.
In regards to subscriptions, Webfilter can still be purchased for SRX240 (SRX240-W-EWF / SRX240-W-EWF-3) but the platform is aging and SRX300 subscriptions are less than half price for even more features (S-SRX300-A2-1 / S-SRX300-A2-3).
The A2 subscription will on top of webfilter also include IPS, App firewall and Antivirus. That said, you should not configure everything on this platform to avoid exhausting all your ressources... but IPS and Webfilter would make sense.
I can only recommend the same approach as Spuluka to migrate to the SRX300 instead of staying on the SRX240H2.