To answer your question directly there is no way of knowing which one is better, the measurements are lacking. There is no way a Subwoofer will play up to 1700hz. Those specifications are nonsense.
Frequency Range of Speakers
Humans hearing range is between 20 and 20 000 hz. 20 000 hz can be written as 20khz which you will see very often. As you age your upper hearing limit will begin to drop at roughly 1khz every 10 years.
You can divide the Frequency range into following categories:
- <20hz Infrasound
- 20 - 80hz audible Bass
- 80 - 300hz Kick bass
- 300 - 1000hz Lower Midrange
- 1000 - 4000hz Upper Midrange
- 4000hz - 20khz Treble
Anything above 20khz is pointless, if you're 50 years of age or older, anything above 15khz will be inaudible.
The producer of your old speaker states it has a frequency range up to 1700hz. Any electrodynamic speaker is capable of producing sounds at 1700hz and even 20khz. The quality and the volume will be severely compromised depending on the model.
First of all, do not ever buy a speaker from a manufacturer that doesn't provide the Thiele-small-parameters (commonly abbreviated as TSP) and a frequency measurement of a speaker. You will be buying junk. Let me give you some examples:
If you click on the link above you can see the TSP listed on the right side on the second page. You need those to know what cabinet to build for the speaker.
The vertical axis is in db. It goes from 70 to 110db. The horizontal axis is a logarithmic scale going from 20 - 20khz. 6db is equal to double volume, double membrane excursion and 4x the power. 10db is equal to roughly 3.3x the volume and 10x the power.
The frequency response graphs are created by playing a frequency sweep from 20hz to 20khz through the speaker at 1 watt power and 1 meter distance in an anechoic chamber. Cheap speaker manufacturers don't bother to measure their speakers. Don't buy those.
The ideal speaker would have a straight line all the way from 20hz to 20khz at 120db. Unfortunately speakers are far from perfect. Speakers are always specialized for a certain bandwidth of frequencies. That's why we see 2 or 3 speakers in a cabinet. In professional high-end PA applications where high-SPL and high sound quality need to be combined a speaker will be optimized for a range of 2 octaves and no more which usually means 5 frequency ranges covered by 5 different speakers. On small speakers it's not economical to do so.
A frequency response peaking at 100db (Commonly abbreviated as sensitivity) means the efficiency of the speaker is 1%. 1% of the electrical energy is converted into acoustic energy, since 120db is equal to 1 watt. The remaining energy is lost as heat, mechanical motion and converted to "wind" due to a poor acoust impedance match with the surrounding air. Something like a car subwoofer usually has an efficiency between 0.05 and 0.1%. Therefore they require a lot of power. Good speakers generally try to maintain a high efficiency.
The frequency response of the RCF LF21x451 is listed as 28 to 1000hz. This usually doesn't mean anything and I still wonder why they supply this "information" at all. Noone in their right mind would use a 21-inch Subwoofer above 200hz. You could cheat and use it up to 150hz but to use it to its full potential you will probably not want to go higher than 80hz. Depending on the cabinet and the setup you can go much, much lower than 28hz, for a home theater system for example.
Precision Devices 2150
Yes the frequency response looks pretty decent until 2khz. However that doesn't mean you should use the speaker for these frequencies.
Normal Hi-Fi Subwoofers are made to extend the bottom range of your current speakers. That usually means playing from 20-50hz or perhaps 20-100hz if your main speakers are small. A Subwoofer should not be playing higher than 100hz because you will be able to localize it. Sounds below 100hz are not localizable.
Large PA Systems generally split the Bass. Large 18 or 21 inch HornLoaded or Direct Radiating Bassreflex will take care of the frequencies up to 80hz. Smaller 15 inch Speakers, mostly Hornloaded in large applications, will take care of the Kickbass up to 350hz.
12 inch Car Subwoofers are usually not meant to go above 80hz. Depending on the model you can cheat and go up to 150hz, perhaps even 200hz but you will get a very muddy and soft kick bass.
Car Subwoofers can be divided into two groups. SPL and SQ. SPL are purely made for maximal volume, SQ are made for optimal sound quality. Also note that since the space inside a car is really small it acts as a pressure chamber. Volume of the bass will be greatly enhanced and you can also get away with much smaller cabinets (which in turn require even more Watts because the smaller volume of air compressed inside the cabinet acts as a force against the membrane creating more resistance the smaller the cabinet) Car Subwoofers can be used at home too but they would require massive enclosures.