Of course it is way faster, the '040 kicks close to 1 instruction per clock cycle average, but you don't see it with the vast majority of amiga software. An '030 needs no libs for compatibility, produces much less heat, and uses less power especially if you don't need the fpu. Which again most amiga software doesn't.
A 040 runs rings around a 030. Just try to load some images and 030 will choke compared to 040. This is very easy to notice. Emulators also run faster on 040 thanks to bigger caches and demos run faster too. Video playing is also a good test.
I never had problems with power or heat with a 040, these "heat" problems sound like urban legends to me. They work fine with a heatsink and if you use it inside a desktop A1200 you may need a very small fan but that's all. 040s do not emit so much heat. Phase5 sold Blizzard040/40 and Cyberstorm040/40 without any fan.
Power requirements are not much higher than 030+fpu.
I jumped from a 68000/7Mhz to an A1200 with Falcon040/25 and I used a BlizzardIV 030/50 the weekends at our local club and I always noticed it way slower. Of course you may find a few applications that perform in a similar way on a 030 but 040 usually crushes 030s. 040 has 4KB+4KB of cache and 030 just 256bytes... even if BlizzardIV memory interface is well designed it can't fight against cache L1. I have/had Amigas with 68000/7, 68020/14, 68030/16, 68030/25, 68030/50, 68040/25, 68040/28, 68040/30, 68040/40, 68060/50, 68060/60, 040/25+PPC, 060/50+PPC... I never had heat problems on my A1200 despiting it carried a 040 and I never had to add stuff to raise it or improve airflow.
Even the slowest 040 I ever touched, a cbm 3640 felt faster in normal use than a BlizzardIV 030/50.
68040.library is included as standard with AmigaOS so he doesn't have to install extra libraries. If he wants to play games WHDLoad is the best piece of software he could use.
This guy is asking for basic advice which I gave. We could get into Mips and stuff or we could give him some simple suggestions on picking an accelerator. With most amiga software imho an '030 is actually better for many people especially beginners.
btw I have had amigas with 68000, 68020, 68030 25mhz and 50mhz, and 68040
If he just wants to play games then he doesn't need much cpu anyway. Just some fastram and whdload. If he's interested on 3D games a 040/28 or faster would be needed.
For demoscene a 060 is almost mandatory for watching last 12 years productions, for apps I would get at least 040/40 and a graphic card.