I don't use Node Red so can't speak to that, other than that one tradeoff is that you need something to run Node Red on (you need a server to run it on, but a "server" can be something as simple as a Raspberry Pi at home). Echo Speaks actually has to be run on something, too, but the installation instructions provide directions for free cloud hosing on your own Heroku setup, though there's nothing stopping you from hosting that elsewhere (including home) if you know how. If all you want is TTS, AlexaTTS is a much smaller app (no external hosting requirements, though you can add an external script for cookie refresh if you want--these are all unofficial options that need some extra work because Amazon doesn't officially support this).
To initiate an Alexa Routine from Hubitat, you technically don't need Echo Speaks, either; there are natively supported ways to do this, like exposing a (virtual) motion or contact sensor to Alexa and (virtually) making it active or open on Hubitat and setting your Alexa routine to trigger based on that. However, some people don't like the extra virtual devices this requires. To do TTS alone, you technically don't need any of the above options, either--this virtual-device workaround works for that too since Routines can do TTS. What you can't do is speak arbitrary text, only whatever you've configured in the routine. So...back to one of those options if that's not what you want, which it sounds like you may (unless Amazon added temperature-sensor-speaking ability recently? I haven't checked).
For TTS, the webCoRE vs. RM distinction doesn't matter. Once you get a TTS device into Hubitat, any app capable of working with that device can make it execute whatever commands the device supports (if the app can send them, which webCoRE and RM are both customizable enough to let you do). Either would work just as well. RM is built in, but webCoRE isn't too difficult to install. "Standard" apps like the Notifications app may even work. I'd start with that or RM personally, but just about anything could work.