I have both. You asked a loaded question, here is my loaded answer. it is just my opinion so others may offer up a different story.
I will start with the Mark V90: Current list price is $3,149 on Mesa's website. It may be harder to get a new one these days since the release of the Mark VII. The V90 is still available on Mesa's website. I had thought about getting another one before the product line gets dropped. There are plenty of them to be found on the used market. $1,650 to $2,400 in your basic black trim.
What I like about the Mark V90:
- it has more power mode configurations available. Some may seem transparent and some are not.
- Lead channel power modes: Pentode and Triode function on the extended Class A tubes. This works on 90W and 45W only. 90W on pentode, 85W on triode, 45W on pentode, 25W-35W on triode. The 10W mode is triode only. Couple that with the full power mode or the variac power mode.
- CH1 and CH2 has a tube tracking switch so you can make use of the Rectifier in the 45W mode for a slight spongy feel. I felt this was more transparent. The MWDR or Roadster it is quite notable when running tube tracking vs diode.
- it has a total of 9 different voicings, clean, fat, tweed, edge, crunch, Mark I, IIC+, IV, and Extreme.
- All channel switching is done with the Mesa old school technology. If you planned on making use of midi, you will need a Mesa Matrix unit as that can be configured to work with the Mark V din cable to replace the footswitch.
- GEQ circuit is in front of the FX loop. Actually, it uses the transistor output circuit to create the FX send level. Some FX units may not work well with the higher impedance source. Signal levels are not all that hot, but they do fall into the line level range +4dBu
- Global volume control and solo boost when using the FX loop in active mode. That gets turned off when the FX loop is switched to hard bypass. This is similar to the MWDR and Roadster as they too can be disabled.
The Mark VII: Current list price is $3,499. Not as many on the used market but can be found for $2,700 up to $3,200 in your basic black trim. The Mark VII is more of a throwback to the Mark III in some ways as well as the JP2C.
- Only has three selectable power modes available, 90W, 45W and 25W. There is no variac power mode with this amp
- The lack of the pentode and triode switch for the lead channel would be nice to have but so far I have not missed that feature.
- Only seven distinct voicings as two are repeated on two channels: Clean, Fat, Crunch VII, IIB, IIC+, IV.
- All channel switching and feature activity is managed by a midi controller.
- Cool feature is you can daisy chain two Mark VII together with a single midi cable and run both amps with one footswitch. Mesa claims the limit is up to 15 amps in total. I have two Mark VII and that is what I am doing so I can run a stereo setup.
- GEQ circuit is located in its traditional position, after the FX loop recovery stage.
- Does not have a global volume or solo boost function.
So how do they compare, there are a few videos out there comparing the two amps in some ways. I tried to do a side by side video. I would not call it all that good as that is not my thing. There are some similarities to each other but yet there are some differences too. You asked about the IIC+ and IV modes. The IIC+ mode on the Mark VII is practically dead on to the JP2C CH2. I can get much the same range of gain with or without the pull switches on the gain or presence controls (JP2C). They are the same in preamp character as I have tried slaving into the JP2C and Mark VII to hear how the power section alters the tone. The IIC+ mode on the Mark V is a bit softer in gain structure. Not as bold and robust as the Mark VII or JP2C I used for comparison as I do not have an 80's IIC to compare too.
Mark IV modes, similar and different. I did compare the Mark V90 to the real deal. Not sure which I liked better as they were about the same. The IV was a B version so there could be some differences with the original format IVA. The Mark VII sort of has more aggressive gain structure, a tad more compression. So far that is my favorite grinder. The second would fall onto the JP2C or the Badlander on crush mode.
Now for the crunch and VII modes, the Mark VII is making use of the lead drive circuit for these modes, but it gets diverted by relays to fit in between the first and second gain stage typically used for the tone stack driver when looking at the IIB, IIC+ and IV circuit paths. Crunch drops the boost stage on the lead drive circuit and hammers the tone stack. It would be similar to the crunch on the Mark V in some ways but it is not the same circuit. It is more to the Badlander crunch in design with the exception the tone stack is plate driven and not pushed into a dc coupled cathode follower circuit (design used by the Roadster and MWDR). The VII mode uses the full lead drive circuit, again it is in front of the plate driven tone stack. Very similar to the Badlander Crush mode. It has more compression than the Badlander but that could be due to the method of how the tone stack is driven. The gain characteristics of crunch sort of fall in between the IIB and IIC+ modes but it can be dialed in to be as aggressive as the IIC+ mode. The VII is like having a blend of the Mark V90 extreme mode and the aggressive nature of the IV mode of the Mark VII. Some tricks you can use on a typical mark amp like boosting the gain and treble and dropping the midrange and bass work quite well on CH3. That trick does not do much for the crunch or VII modes.
If either of them interest you, best to sit down with one and find out in person how the sound when in the room. Recorded sounds from videos and such may be different than it is in the room through a speaker cab. Never explored the cab clone IR yet.