Honest answer, I wouldn't buy a HR monitor if I were you.
And that's coming from someone who trained with one for many years and was coached to HR targets while I trained. It took a long time to come around but in the end I found it did more to hold my training back than to advance my training. Far too many of my workouts were dictated by NOT exceeding a particular HR and I got pretty good at going slow or holding moderate pace but still struggled when the road tipped up or the pace got hot in races.
About seven seasons ago I started training with a power meter and for a couple of seasons continued to wear the HR strap and record both power and HR data. The charts from that training and racing made things very clear. More often than not HR became my limiter and not something that encouraged doing the work. When I ignored HR I trained at higher power and over time my results improved substantially. In time I stopped wearing the HR strap altogether.
If I was starting from scratch and a power meter was not in the budget I'd train as riders did before any of these gadgets were invented. I'd train by tuning into my own Rate of Perceived Exertion (RPE) or basically tune into how I felt, how heavily I was breathing, basically how comfortable or uncomfortable my efforts were during training. On focused days I'd pick durations of interest that target specific systems (e.g. 3 to 6 minute hills or sections of flatter road for VO2 Max intervals, 20 to 30 or even 60 minute sections of road for Threshold time trial intervals, 1 to 3 hour open road rides for Tempo work, longer all day rides for long Endurance work, etc.) and ride those efforts hard but paced to finish the duration without totally blowing up or fading. Yep, it takes some time to learn that pacing and at the beginning folks will tend to hit the efforts too hard and blow up or go too easy and realize they could have ridden harder and faster. But it self corrects pretty fast if you just pay attention to how you feel, and your speed as you do these efforts.
The ironic thing is when I first started getting serious about cycling on the college racing team an older local athlete made almost exactly that same advice. But that was in the mid '80s just as the first wristwatch HR monitors hit the consumer market. And I jumped on the tech bandwagon like everybody else and was sure that wristwatch that recorded my HR was the ticket to much better training and faster racing. It didn't work that way for me and from the coaching I've done in recent years I see that I'm not the only one.
No doubt many folks have used HR monitors to assist their training and many no doubt have been very successful doing so. But IME the data they provide can be very misleading and ultimately somewhat redundant to RPE. So yeah your heart is beating faster or slower today for a similar effort but does that mean back off if you feel good or perhaps slow down if you feel bad? If you tune into your body and develop pacing skills you'll know what to do in those situations regardless of what the HR meter says. And tracking HR over time doesn't really tell you anything about fitness or progress. Sure matching that HR to time up a local climb or typical rolling speeds might, but the time up the climb or numbers on your speedometer already tell you when you're getting faster.
So I personally wouldn't recommend buying a HR monitor. If you have the budget I might suggest a power meter but it depends on how much you're willing to invest in learning how it can help you and how much structured training you intend to do. At least the PM gives you absolute measurements on progress (if you can sustain more power for durations of interest you're getting fitter in a measurable way not subject to the wind direction or the terrain or the road conditions as simply measuring speed often is). But from your description above it doesn't sound like you're in the market for a PM in which case I'd go with a standard cycling computer and RPE.
-Dave
